For the first time, Colombia will have a leftist president.

Gustavo Petro, a former rebel and a longtime senator who has pledged to transform the country’s economic system, has won Sunday’s election, according to preliminary results, setting the third largest nation in Latin America on a radically new path.

Mr. Petro, 62, received more than 50 percent of the vote, with more than 99 percent counted Sunday evening. His opponent, Rodolfo Hernández, a construction magnate who had energized the country with a scorched-earth anti-corruption platform, just over 47 percent.

Shortly after the vote, Mr. Hernández conceded to Mr. Petro.

“Colombians, today the majority of citizens have chosen the other candidate,” he told his supporters in Bucaramanga. “As I said during the campaign, I accept the results of this election.”

Just over 58 percent of Colombia’s 39 million voters turned out to cast a ballot, according to official figures.

Mr. Petro’s victory reflects widespread discontent in Colombia, a country of 50 million, with poverty and inequality on the rise and widespread dissatisfaction with a lack of opportunity, issues that sent hundreds of thousands of people to demonstrate in the streets last year.

“The entire country is begging for change,” said Fernando Posada, a Colombian political scientist, “and that is absolutely clear.”

The win is all the more significant because of the country’s history. For decades, the government fought a brutal leftist insurgency known as the Revolutionary Armed Forces of Colombia, or FARC, with the stigma from the conflict making it difficult for a legitimate left to flourish.

But the FARC signed a peace deal with the government in 2016, laying down their arms and opening space for a broader political discourse.

Mr. Petro had been part of a different rebel group, called the M-19, which demobilized in 1990, and became a political party that helped rewrite the country’s constitution.

Both Mr. Petro and Mr. Hernández beat Federico Gutiérrez, a former big city mayor backed by the conservative elite, in a first round of voting on May 29, sending them to a runoff.

Both men had billed themselves as anti-establishment candidates, saying they were running against a political class that had controlled the country for generations.

Among the factors that most distinguished them was how they viewed the root of the country’s problems.

Mr. Petro believes the economic system is broken, overly reliant on oil export and a flourishing and illegal cocaine business that he said has made the rich richer and poor poorer. He is calling for a halt to all new oil exploration, a shift to developing other industries, and an expansion of social programs, while imposing higher taxes on the rich.

“What we have today is the result of what I call ‘the depletion of the model,’” Mr. Petro said in an interview, referring to the current economic system. “The end result is a brutal poverty.”

His ambitious economic plan has, however, raised concerns. One former finance minister called his energy plan “economic suicide.”

Mr. Petro will take office in August, and will face pressing issues with global repercussions: Lack of opportunity and rising violence, which have prompted record numbers of Colombians to migrate to the United States in recent months; high levels of deforestation in the Colombian Amazon, a critical buffer against climate change; and growing threats to democracy, part of a trend around the region.

He will face a deeply polarized society where polls show growing distrust in almost all major institutions.

Mr. Petro could also reshape Colombia’s relationship with the United States.

For decades, Colombia has been Washington’s strongest ally in Latin America, forming the cornerstone of its security policy in the region. During his campaign, Mr. Petro promised to reassess that relationship, including crucial collaborations on drugs, Venezuela and trade.

In the interview, Mr. Petro said his relationship with the United States would focus on working together to tackle climate change, specifically halting the rapid erosion of the Amazon.

“There is a point of dialogue there,” he said. “Because saving the Amazon rainforest involves some instruments, some programs, that do not exist today, at least not with respect to the United States.”

Megan Janetsky contributed reporting from Bucaramanga, Colombia, and Sofía Villamil and Genevieve Glatsky contributed reporting from Bogotá.

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The capital’s streets were filled with the sound of honking cars and motorcycles as drivers rolled down their windows and cheered, and a smattering of fireworks filled the cloudy evening skies.

Inside the arena, supporters with Colombian flags wrapped around their shoulders shouted, “yes he could, yes he could!” Party lights flashed. Supporters jumped up and down.

In the rafters, a banner with red lettering stretched across dozens of seats: “The people are bigger than their leaders,” it read.

Mr. Petro’s speech was preceded by the entrance of the Indigenous guard, a traditional security unit whose members hold staffs meant to represent peace and strength.

Mr. Petro took the stage flanked by his vice-presidential pick, Francia Márquez, and three of his children. The packed stadium went wild, with people standing on chairs and holding phones aloft.

“This story that we are writing today is a new story for Colombia, for Latin America, for the world,” he said. “We are not going to betray this electorate.”

He pledged to govern with what he has called “the politics of love,” based on hope, dialogue and understanding.

About 250 miles north of Bogotá, chants of “yes we can,” echoing through the streets, faded to a sad silence in Rodolfo Hernández’s hometown, Bucaramanga, as the election results rolled in.

Supporters left blue and red “Rodolfo” signs scattered across the concrete ground, and let bunches of campaign balloons fly into the sky, signaling the end of a quick but feverish campaign.

As supporters trickled out of Mr. Hernández’s campaign event, a 46-year-old shoe vendor, Omar Quintero, stood to the side in a bright yellow Colombian soccer jersey sadly smoking a cigarette.

“To me, this means we are under the rule of the left and that is what we didn’t want in Colombia,” Mr. Quintero said. “This is a new experience and an uncertainty because we don’t know what will happen.”

He said that while he worries, he also hopes Mr. Petro follows through on his anti-corruption message, something shared with Mr. Hernández.

Shortly after the election results were announced, Mr. Hernández conceded to Mr. Petro.

“Colombians, today the majority of citizens have chosen the other candidate,” he said. “As I said during the campaign, I accept the results of this election.”

Gloria Jaime Sanchez, 47, wore a hat with the candidate’s face on it, and a dozen “Rodolfo” buttons pinned to her shirt.

She said she woke up excited and hopeful that Mr. Hernández, who once governed her city, would win the election. But now she said she felt a sense of dread.

“With this gentleman, I am not in the spirit. I am not in the mood. I’m very sad,” she said, walking off with a Colombian flag draped over one shoulder.

As mayor, he called himself “the king,” punched a councilman who offended him and told a city employee pushing him to follow the rules that he’d wipe his own buttocks with the law.

Rodolfo Hernández, a 77-year-old businessman and former mayor, emerged as Colombia’s most disruptive presidential candidate in decades, electrifying voters with a single-issue “drain the swamp” message amplified by a team of social media wizards who have made him a TikTok star, allowing him to circumvent the trappings of conventional campaigns.

During the campaign, Mr. Hernández avoided most debates and held few public events, favoring interviews with friendly media and live streams run by his allies. Yet he energized broad swaths of the electorate, with his advisers saying that he has understood the moment.

He was one of two remaining candidates in Sunday’s election for president of the third largest nation in Latin America, with the winner taking control at a pivotal moment in the country’s history.

“What the Colombian people really want is to rescue the entire public administration from the clutches of politicians,” he told The New York Times. “I embody that.”

The Trump-like figure was dismissive of his tendency to offend, including calling Venezuelan women a “factory for making poor children” and declaring himself a follower of the “great German thinker” Adolf Hitler.

“I say what I feel,” Mr. Hernández said. “I’m not interested in the aftereffect.”

Still, he has clarified that he meant to say Albert Einstein.

As a candidate, Mr. Hernández promoted himself as a paragon of democracy, a successful businessman who makes good on promises and cares for the poor. But a trip to Bucaramanga, a mountain-fringed city where he built his empire and once served as mayor, reveals a different picture.

Mr. Hernández’s supporters describe him as a savior who erased the city’s deficit, renegotiated contracts to benefit taxpayers and broke a cycle of political favors that had turned Bucaramanga into a capital of corruption.

His critics called him a danger to democracy, an evangelist of a brutal capitalism that would ruin the nation and a man with few firm policy ideas who would do whatever it took to get his way.

Today, Mr. Hernández faces corruption charges in that case, accused of pushing subordinates to ensure a specific company won a deal with the city. According to the inspector general’s office, that contract could have earned his son significant money.

Mr. Hernández’s trial begins July 21. He has said he is innocent.

“I didn’t steal anything,” he said. “That’s why I’m calm, with a clear conscience.’’

Carlos Buitrago contributed reporting from Bucaramanga and Piedecuesta, and Genevieve Glatsky from Bogotá.

And his ascent was, in no small part, propelled by the biggest, loudest and possibly angriest youth electorate in Colombia’s history, demanding the transformation of a country long cleaved by deep social and racial inequality.

There are now nearly nine million Colombian voters 28 or younger, the most in history, and a quarter of the electorate. They are restive, raised on promises of higher education and good jobs, disillusioned by current prospects, more digitally connected and arguably more empowered than any previous generation.

Today’s younger generation is grappling with 10 percent annual inflation, a 20 percent youth unemployment rate and a 40 percent poverty rate. Many say they feel betrayed by decades of leaders who have promised opportunity but delivered little.

Young people led anti-government protests that filled the streets of Colombia last year, dominating the national conversation for weeks. At least 46 people died — many of them young, unarmed protesters and many at the hands of the police — in what was known as the “national strike.”

In a June poll by the firm Invamer, more than 68 percent of voters ages 18 to 24 and nearly 61 percent of voters ages 25 to 34 said they were planning to vote for Mr. Petro.

In contrast, just over 30 percent of people ages 18 to 24 and just over 36 percent ages 25 to 34 said they would vote for Mr. Hernández.

The election came at a difficult moment for the country. Polls show widespread dissatisfaction with the government of the current president, Iván Duque, and frustration over chronic poverty, a widening income gap and insecurity, all of which have worsened during the pandemic.

Some analysts expected young people to vote in large numbers, energized not just by Mr. Petro but by his running mate, Francia Márquez, 40, an environmental activist with a gender, race and class-conscious focus who will be the country’s first Black vice president.

“The TikTok generation that is very connected to Francia, that is very connected to Petro, is going to be decisive,” said Fernando Posada, 30, a political analyst.

But many young voters were skeptical of Mr. Petro’s ability to deliver on his promises.

In Fusagasugá, Nina Cruz, 27, a cafe worker, said Mr. Petro would fail Colombia’s struggling families, and she was particularly repulsed by his past as a member of a leftist rebel group.

The country has a long history of violent militias that claim to help the indigent — and end up terrorizing them.

“What he is saying is: ‘I’m going to help the poor,’” she said. “That’s a total lie.”

For some Colombian voters, his past was a source of concern after decades of armed conflict. For others, it offered a sign of hope for one of most inequitable countries in Latin America.

The M-19 was born in 1970 as a response to alleged fraud in that year’s presidential elections. It was far smaller than the country’s main guerrilla force, the Revolutionary Armed Forces of Colombia, or FARC, which was Marxist and sought haven in Colombia’s jungles and rural areas.

The M-19 was an urban military group formed by university students, activists and artists who wanted to topple a governing system they believed failed to bridge a chronic divide between the rich and the poor.

“The M-19 was born in arms to build a democracy,” Mr. Petro told The New York Times in an interview.

It originally tried to promote a Robin Hood image, robbing milk from supermarket trucks to distribute in poor neighborhoods and, in a symbolic act of rebellion, stole a sword from a museum that Simón Bolívar used in Colombia’s war for independence.

Mr. Petro, 62, joined the group when he was 17 and an economics student, dismayed by the poverty he witnessed in the town where has living, outside Bogotá, the capital.

While the M-19 was less brutal than other rebel groups, it did orchestrate what is considered one of the bloodiest acts in the country’s recent history: the 1985 siege of Colombia’s national judicial building that led to a battle with the police and the military, leaving 94 people dead.

The group also stole 5,000 weapons from the Colombian military and used kidnapping as a tactic to try to wrest concessions from the government.

Mr. Petro, who spent 10 years in the M-19, largely stockpiled stolen weapons, said Sandra Borda, a political science professor at the University of the Andes in Bogotá.

“What’s key is that he wasn’t part of the main circle who made the decisions in M-19. He was very young at that moment,” she said. “He didn’t participate in the most important operations of the M-19, the military operations.”

At the time of the justice building takeover, Mr. Petro was in prison for his involvement with the group and he has described being beaten and electrocuted by the authorities.

The group eventually demobilized in 1990, which was considered one of the most successful peace processes in the country’s long history of conflict. It turned into a political party that helped rewrite the country’s constitution to focus more on equality and human rights.

Mr. Petro ran for Senate as a member of the party, launching his political career.

Sofía Villamil and Julie Turkewitz contributed reporting from Bogotá.

Francia Márquez, an environmental activist from the mountainous department of Cauca in southwestern Colombia, has become a national phenomenon, mobilizing decades of voter frustration, and becoming the country’s first Black vice president on Sunday, as the running mate to Gustavo Petro.

The Petro-Márquez ticket won Sunday’s runoff election, according to preliminary results. Mr. Petro, a former rebel and longtime legislator, will become the country’s first leftist president.

The rise of Ms. Márquez is significant not only because she is Black in a nation where Afro-Colombians are regularly subject to racism and must contend with structural barriers, but because she comes from poverty in a country where economic class so often defines a person’s place in society. Most recent former presidents were educated abroad and are connected to the country’s powerful families and kingmakers.

Despite economic gains in recent decades, Colombia remains starkly unequal, a trend that has worsened during the pandemic, with Black, Indigenous and rural communities falling the farthest behind. Forty percent of the country lives in poverty.

Ms. Márquez, 40, chose to run for office, she said, “because our governments have turned their backs on the people, and on justice and on peace.”

She grew up sleeping on a dirt floor in a region battered by violence related to the country’s long internal conflict. She became pregnant at 16, went to work in the local gold mines to support her child, and eventually sought work as a live-in maid.

To a segment of Colombians who are clamoring for change and for more diverse representation, Ms. Márquez is their champion. The question is whether the rest of the country is ready for her.

Some critics have called her divisive, saying she is part of a leftist coalition that seeks to tear apart, instead of build upon, past norms.

She has also never held political office, and Sergio Guzmán, director of Colombia Risk Analysis, a consulting firm, said that “there are a lot of questions as to whether Francia would be able to be commander in chief, if she would manage economic policy, or foreign policy, in a way that would provide continuity to the country.”

Her more extreme opponents have taken direct aim at her with racist tropes, and criticize her class and political legitimacy.

But on the campaign trail, Ms. Márquez’s persistent, frank and biting analysis of the social disparities in Colombia cracked open a discussion about race and class in a manner rarely heard in the country’s most public and powerful political circles.

Those themes, “many in our society deny them, or treat them as minor,” said Santiago Arboleda, a professor of Afro-Andean history at Simón Bolívar Andean University. “Today, they’re on the front page.”