BOGOTÁ, Colombia—Far-right populist Abelardo de la Espriella was narrowly elected president of Colombia on Sunday, setting the country on a collision course with the cocaine-trafficking networks and criminal gangs that have extended their reach across drug-producing regions.Abelardo de la Espriella after casting his vote on Sunday.A flamboyant 47-year-old lawyer who flaunts his wealth and holds U.S. citizenship, de la Espriella received 12.9 million votes, or 49.6% of the ballots cast, by pledging to dismantle armed groups that have surged in recent years.His rival, Ivan Cepeda, a far-left senator who promised to deepen President Gustavo Petro’s leftist policies, received 48.7% of ballots in this country of 53 million. More than 26 million Colombians voted, 63% of registered voters, with a difference of only 247,000 between the two candidates.With media outlets in Colombia calling the victory for de la Espriella, his supporters honked horns in the capital and gathered to celebrate.But with such a tight margin between the two candidates, Petro said on his X account that “no one can be declared president yet.” He called on an official canvass of votes by electoral authorities to determine the winner.Other prominent figures in the president’s movement had warned that Cepeda supporters would rise up in the streets should he lose.“Let business leaders be warned,” Gustavo Bolivar, a prominent Petro aide, said last week, “if the far right wins, this country will erupt in flames.”Sunday’s vote tally is technically preliminary—a rapid and highly accurate count that in past elections served as the final say on the winner. But it isn’t official until electoral authorities ratify tally sheets from individual stations across Colombia and dozens of countries where Colombian immigrants voted, said Hernan Penagos, who leads the electoral board.“Once the official vote count is completed and the corresponding verifications have been made, we will recognize the official result,” Cepeda told his followers Sunday night.De la Espriella said on the Caracol television network that “we defeated the regime, the political establishment, and the usual elites. We won against all odds.”He has pledged a sharp break with Petro’s leftist rule, in which Colombia ratcheted up cash transfers to the poor, started land reform to get farms into the hands of landless peasants and embarked on talks to get armed militias to disarm.“I will be relentless against those who seek to destroy Colombia,” de la Espriella said in a recent speech.Calling himself “the Tiger,” de la Espriella has directed his ire at some of Colombia’s most powerful armed groups and notorious warlords.“I declare you military targets, and I’m going to take you down,” he said. “The Tiger will make Colombia a miracle nation.”Many Colombians have been disillusioned by the growth in armed groups, violence and the cocaine trade under Petro, himself a former leftist guerrilla.Farms that grow coca, the main ingredient in cocaine, now cover 625,000 acres, nearly the size of Rhode Island. That is 55% more than in 2000, when the U.S. began providing Colombia billions in equipment and training to roll back drug production and Marxist guerrillas.Trafficking networks have used that coca to produce so much cocaine that American officials say the price of the drug in the U.S. might be at a historic low. Heavily armed militias—hybrid groups that are part insurgencies, part drug syndicates—have in four years doubled in size to some 25,000 fighters, spreading across hundreds of towns.Those results have dismayed U.S. officials and led President Trump to openly support de la Espriella, who pledges to build mega-prisons like another politician who enjoys U.S. backing, President Nayib Bukele of El Salvador.After de la Espriella won a first round of voting in May, Trump called Cepeda a “radical leftist Marxist” and offered his “complete and total endorsement” of the conservative candidate in a Truth Social post. On Wednesday, Trump called de la Espriella “Smart, Strong, and Tough” and wrote that “because of his competence and love for his Country, he will have the full support and strength of the United States.”Cepeda had been an architect of the president’s so-called Total Peace plan to get cocaine-trafficking groups to lay down their arms through simultaneous negotiations. Aside from the demobilization of 95 fighters from one group in southern Putumayo province this past week, the militia organizations grew stronger, giving de la Espriella a signature campaign issue.De la Espriella has also had the backing of Wall Street, pledging he would reduce spending by 40%, close nine ministries and fire 700,000 state workers. With only four senators and one representative in congress, de la Espriella faces challenges in getting the political backing for his overhaul, said Sergio Guzmán, director of the Colombia Risk consulting firm.“Their expectations will be constrained,” Guzman said of investors.An outsider with no party, de la Espriella is an unusual caretaker for a country battling drug gangs.He is a defense attorney who became rich representing underworld figures, including right-wing paramilitary commanders who trafficked cocaine to U.S. cities and Alex Saab, the alleged money launderer for former Venezuelan strongman Nicolás Maduro. Venezuela’s government recently extradited Saab to the U.S. to face charges.Write to Juan Forero at juan.forero@wsj.com