Colombia's election stations opened for voting Sunday in a presidential race filled with uncertainty, as ex-guerrilla Gustavo Petro and millionaire businessman Rodolfo Hernandez vie for power in a country saddled with widespread poverty, violence and other woes.

Abstention is expected to be high as voters face a stark choice between electing their first-ever left-wing president or plumping for a maverick outsider dubbed the Colombian Donald Trump.

In Bogota, outgoing President Ivan Duque opened voting for Colombia's 39 million voters at 8:00 am (1300 GMT).

Polls will close at 4:00 pm, with early results expected a couple of hours after that.

Hernandez was amongst the early voters in the northern city of Bucaramanga, where he was mayor from 2016 to 2019.

"What we have now in the country are questions, uncertainties," Patricia Ines Munoz, an expert at the Pontifical Javerian University, told AFP.

It has been a tense campaign, with death threats against several candidates ahead of the first round last month, when Colombia's traditional conservative and liberal powers were dealt a chastening defeat.

"These are the tightest elections in the country's recent history," said the Sunday edition of the El Tiempo daily.

There are fears a tight result could spark post-election violence and 320,000 police and military have been deployed to ensure security.

The successor to unpopular conservative Duque will have to deal with a country in crisis, reeling from the coronavirus pandemic, recession, a spike in drug-trafficking-related violence and deep-rooted anger at the political establishment.

Almost 40 percent of the country lives in poverty while 11 percent are unemployed.

That anger spilled over into mass anti-government protests in April 2021 that were controversially met by a heavy-handed response from the security forces.

Opinion polls in the lead up to the election have been inconclusive, although abstention is expected to be 45 percent with up to another five percent undecided.

"I feel very bewildered," Camila Araque, a 29-year-old lawyer in Bogota, told AFP. "I don't like either of the two options as president." 

'Understandable hysteria'

Michael Shifter, from the Inter-American Dialogue think tank, says voters "are trying to figure out who is the lesser of two evils."

Petro comfortably topped the first round of voting with 40 percent, 12 points ahead of Hernandez.

But Petro's past as a radical leftist urban guerrilla in the 1980s -- during which time he spent two years in prison on arms charges -- has left many Colombians fearful.

He has been in politics since his M-19 group made peace with the state in 1990 and formed a political party.

"The worry comes from the experience of leftist governments in the region," said Munoz, "not just among citizens but also the business and economic sectors."

Some believe the former mayor of Bogota would turn Colombia into another authoritarian populist socialist state like neighbouring Venezuela.

"It borders on hysteria," said Shifter, but "it's understandable because... more than any other Latin American country, the Venezuela tragedy and nightmare has impacted Colombia," creating a "terror" that they are next.

Petro, 62, says the country needs social justice to build peace after a six-decade multi-faceted conflict involving leftist rebels, the state, right-wing paramilitaries and drug cartels.

"That is to say less poverty, less hunger, less inequality, more rights. If you don't do this, the violence proliferates," he told Caracol Radio on Friday.

Petro, who is popular with many young people, named environmentalist feminist Francia Marquez, 40, as his running mate.