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 about 40 per cent support, ten points below what he would need to secure the presidency without a June second round.

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 Gutierrez, the centre-right former mayor of Medellin, who has around 25 per cent support.

Gutierrez has emphasised his own plans for a basic income for five million households, economic growth of five per cent per year, and more efficient government spending 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 the leftist's economic plans, including a ban on new oil and gas projects, will ruin Colombia's economy.

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

Polls will open at 8am on Sunday and close at 4pm, with results expected about four hours after closing.

Australian Associated Press