Gustavo Petro, the former mayor of Bogota and ex-rebel fighter, has become Colombia’s first left-wing president.
Mr Petro, a current senator, defeated the right-wing construction magnate Rodolfo Hernández in Sunday’s run-off election.
Figures show he took 50.5% of votes, defeating his millionaire rival by a close margin of around 700,000 ballots.
The result marks a major change for the country, which for decades has been led by moderates and conservatives.
The vote was held amid widespread discontent at the way the country has been run, and there were anti-government protests last year in which dozens of people died.