Petro's hard-fought victory in June elections brought Colombia, long ruled by a conservative elite, into an expanding left-wing fold in Latin America.
Petro has promised to raise taxes on the rich, invest in health care and education, and reform the police after a brutal crackdown on anti-inequality protests last year.
Colombia was one of around 60 countries to reject Venezuela’s 2018 presidential election, which was boycotted by the opposition.
The tragedy occurred at a prison facility in the city of Tulua during an escape attempt followed by a fire, Colombian authorities say.
Pre-vote polls show former rebel Gustavo Petro ahead so far as Colombians vote for their next leader in a country facing rising poverty and insecurity.
Nearly 39 million of the country’s 50 million inhabitants are eligible to vote to renew the upper and lower houses of Congress and to elect a shortlist of presidential candidates.
President Ivan Duque says Euclides Espana alias Jhonier was killed in a clash with the military, calling it "one of the greatest blows" dealt to the FARC dissidents.
Civilians were "likely" among the 23 casualties from clashes between members of last recognised guerrilla group, the ELN, and dissidents of the FARC rebel movement, in eastern Arauca state, officials say.
Tax reform is the issue that in April sparked mass nationwide protests that left more than 60 people dead and the Colombian government is facing accusations of use of excessive force.
Colombian unions, student groups and civil society members say they will continue their demonstrations, seeking social change amid intermittent talks between the government and strike organisers.
Colombians have protested across the country against President Ivan Duque's government, which they feel has long ignored their needs.
Since April 28, hundreds of thousands of Colombians have taken to the streets to show their frustrations against the government after poverty and violence soared during the pandemic.
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