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.
Ex-guerrilla and former mayor Gustavo Petro set to be sworn in as Colombia's first-ever leftist president, with plans for profound reforms in a country beset by economic inequality and drug violence.
He will be inaugurated at 3:00 pm local time (2000 GMT) on Sunday in front of a host of international guests. Colombian presidents serve only one term.
The former senator, 62, takes over from the deeply unpopular Ivan Duque for a four-year term during which he will enjoy support from a left-leaning majority in Congress.
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 that could be consolidated in October with a likely victory for Luiz Inacio Lula da Silva in Brazil.
At a ceremony in Bogota on the eve of his inauguration, Petro said his government would aim to "bring to Colombia what it has not had for centuries, which is tranquility and peace."
On the campaign trail, Petro had 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 that was internationally condemned.
He has vowed to suspend oil exploration, to promote clean energy and to reactivate diplomatic and commercial relations with the government of Nicolas Maduro in Venezuela, suspended since 2019.
Petro's presidency is historic in another sense, too: by his side will be the country's first-ever Afro-Colombian woman vice-president, environmental and women's rights activist Francia Marquez, 40.
The pair will grapple with an economy reeling from the coronavirus pandemic, a spike in violence and deep-rooted anger at the political establishment that culminated in last year's protests.
Almost 40 percent of Colombia's 50 million people live in poverty, while 11.7 percent are unemployed. Inflation reached 10.2 percent year-on-year in July.
Petro, who in his youth was a member of the M-19 urban guerrilla group, has promised to implement outstanding provisions of the 2016 peace agreement that saw the rebel FARC movement lay down arms after nearly six decades of civil conflict.
The former mayor of Bogota has also vowed to pursue negotiations with the National Liberation Army (ELN) armed group.
Despite the FARC disbanding to become a political party, Colombia has seen a surge in violence as thousands of dissidents battle the ELN and powerful cartels for control of drug fields, illegal gold mines and lucrative smuggling routes.
Petro has proposed allowing armed groups to hand themselves over in exchange for some form of amnesty.