Colombians voted on Sunday to choose a new president from among a former rebel promising generous social programs, a center-right candidate warning against a leftist economic mode, and an eccentric business magnate.

Gustavo Petro, a leftist former mayor of Bogota and member of the M-19 guerrilla group and current senator, is consistently leading opinion polls with around 40% support, 10 points below what he would need to secure the presidency without a second round in June.

The 62-year-old has attracted backing on promises to redistribute pensions, offer free public university and battle deep inequality.

His main opponent is Federico 'Fico' Gutierrez, the center-right former mayor of Medellin, who has around 25% support.

Gutierrez has emphasized his own plans for a basic income for 5 million households and economic growth of 5% per year in response to accusations he is an ideological successor to unpopular President Ivan Duque.

Gutierrez, 47, has said Petro is a threat to democracy and warned that the leftist's economic plans, including a ban on new oil and gas projects, will ruin Colombia's economy.

Polls closed at 4 p.m. (2100 GMT), with definitive results expected about four hours later.

Casting her ballot for Petro in Bogota's Chapinero neighborhood was Ana Maria Sierra, who said free university education and the environment were among her priorities.

"One must be more empathetic with the needs of others," the 27-year-old waitress said.

Youth are a key demographic for Petro, who has about 50% support among the country's youngest voters.

His campaign has ramped up efforts to encourage youth turn-out in the last few days, acknowledging that young people may need an extra push to vote over a holiday weekend.

"If everyone who says they're going to vote for Petro, the young people, votes, we'll win in the first round," Alfonso Prada, Petro's top advisor, told journalists on Saturday. "It's a very important demographic for us."

Others told Reuters they were backing Gutierrez on his promise to support businesses.

"I'm betting on Fico, he seems to me the best option to preserve the economy," said 58-year-old pawn shop owner Pablo Guevara as he voted in the southern Bogota neighborhood of Villa del Rio. "I couldn't convince my wife or my kids - they went for Petro."

Polling third in the six-way race is construction magnate and former mayor of Bucaramanga Rodolfo Hernandez, with about 20% support.

Hernandez, who is running independently, is known for whimsical social media videos, including of him riding an electric scooter, and anti-corruption promises.

The 77-year-old is himself facing an ongoing investigation into whether he intervened in a tender to benefit a company his son lobbied for. He has denied wrongdoing.

The national police reported no major public order problems, but they did share a video of a polling place in northern Sucre province flooded by heavy rains, where officers were helping citizens climb out of a boat onto an elevated platform to cast their ballots.

The country's Registrar has said there is no possibility of electoral fraud, after candidates repeatedly expressed concerns about irregularities during March legislative elections which electoral officials classed as procedural errors.