Gustavo Petro, a leftist former member of the M-19 guerilla movement who has vowed profound social and economic change, has won Colombia's presidency, the first progressive to do so in the country's history.

Mr Petro beat real estate millionaire Rodolfo Hernández with a margin of some 716,890 votes, winning 50.5 per cent of the vote compared with Mr Hernández's 47.3 per cent.

"Today is undoubtedly a historic day. We are writing history at this moment. A new history for Colombia, for Latin America, for the world," said Mr Petro.

Some 22.6 million people voted, with 2.3 per cent handing in protest votes, backing neither candidate.

New path for Colombia 

Colombians voted amid widespread discontent over rising inequality, inflation and violence. 

Those factors led voters in the election's first round last month to turn their backs on the long-governing centrist and right-leaning politicians and chose two outsiders.

Mr Petro's showing marks a drastic change in presidential politics for a country that has long marginalised the left for its perceived association with armed conflict.

He pledged to fight inequality with free university education, pension reforms and high taxes on unproductive land. 

Mr Petro's proposals — especially a ban on new oil projects — have startled some investors, though he has promised to respect current contracts.

A fragmented congress, where a dozen parties have seats, will act as a check on Mr Petro's proposals, said Daniela Cuellar of FTI Consulting.

"Colombia's institutional strength and rule of law appear sufficiently robust for the country to maintain economic stability," she said.

"Even if he tries to pass radical reforms, he does not have the congressional support to implement them."

Colombians voting for change 

Supporter Alejandro Forero, 40, who uses a wheelchair, cried as results rolled in at the Petro campaign celebration in Bogota.

"Finally, thank God. I know he will be a good president and he will help those of us who are least privileged. This is going to change for the better," said Mr Forero, who is unemployed.

This campaign was Mr Petro's third presidential bid and his victory adds the Andean nation to a list of Latin American countries that have elected progressives in recent years.

Mr Petro's running mate Francia Márquez, a single mother and former housekeeper, will be the country's first Afro-Colombian woman vice-president.

"Today I'm voting for my daughter — she turned 15 two weeks ago and asked for just one gift: that I vote for Petro," said security guard Pedro Vargas, 48, in Bogota's south-west.

Surprise contender Rodolfo Hernández 

Mr Hernández, who served as mayor of Bucaramanga, was a surprise contender in the run-off.

He had promised to shrink government and to finance social programs by stopping corruption, as well as provide free narcotics to addicts.

Despite his anti-graft rhetoric, Mr Hernández is under a corruption investigation over allegations he intervened in a trash management tender to benefit a company his son lobbied for. He has denied wrongdoing.

Mr Hernández, who refers to himself as the "king of TikTok", ran a non-traditional campaign focused mostly on social media, repeatedly cancelling media interviews and barely appearing in public in the 10 days leading up to the vote.

He conceded defeat in a short video on social media.

"As I said during the campaign, I accept the results of this election," Mr Hernández said, adding he hopes Mr Petro will be loyal to his anti-corruption promises.

Dejected Hernández supporters were throwing away campaign balloons outside his headquarters in Bucaramanga.

Reuters/AP