The 62-year-old former senator, who left the armed rebellion three decades ago, will be sworn in around 3 p.m. local time (2000 GMT) in front of a large delegation of international guests.
"The first government that we hope will be a government of peace is about to begin. We hope it can bring to Colombia what it has not had for centuries, namely the tranquility and peace," Petro said on Saturday in Bogota during an enthronement ceremony among indigenous, Afro-Colombian and peasant peoples.
"Here begins a government that will fight for environmental justice", he also launched, eager to make peace, reductions in inequalities and ecology the primary battles of his government.
The former opposition leader for two decades takes office with a battery of reforms in mind that have raised high expectations among his supporters since his victory on June 19.
Alongside her, environmentalist Francia Marquez, 40, will be sworn in as the first Afro-Colombian vice president of a nation that has historically been ruled by white male elites.
Mr Petro starts from an "enviable position, with a large majority in parliament and enjoys, at street level, support that no government has had in recent years", AFP told AFP. analyst Jorge Restrepo of the Resource Center for Conflict Analysis (Cerac).
- Financing the reforms -
Gustavo Petro has formed a plural government, with women at the head of several portfolios, with the mission of advancing the reforms which will begin their legislative journey on Monday.
In search of resources to finance social reform plans, bills intend to increase taxes on the richest, improve their collection or tax sugary drinks.
But "the level of debt and fiscal deficit that we found are critical," said Daniel Rojas, one of the coordinators of the transition commission with the government of his predecessor Ivan Duque (2018-2022).
"I will fight for social justice in one of the most aberrant societies of inequality on Earth," he said Saturday.
While Colombia's economy has recovered from the pandemic and returned to growth, July's 10.2% year-on-year inflation, unemployment (11.7%) and 39% poverty make President Petro's challenges still taller.
"People expect some of the changes promised during the campaign to be implemented quickly, which, added to the economic situation (...) generates an atmosphere of tension", warns Patricia Muñoz, political scientist at the Javeriana University.
- Armed peace -
Internationally, Mr. Petro will reactivate the diplomatic and trade relations severed since 2019 with neighboring Venezuela of Nicolas Maduro, and seek support to resume peace talks with the National Liberation Army (ELN), the last guerrilla recognized in the country.
Although the peace agreement with the Revolutionary Armed Forces of Colombia (FARC) in 2016 has reduced violence, Colombia has not yet extinguished the continent's last internal armed conflict.
In addition to the ELN, powerful gangs of drug traffickers such as the Clan del Golfo, led by Baron "Otoniel" extradited this year to the United States, impose their law in several regions of the country.
And FARC dissidents are also challenging the state with resources from illegal mining and, above all, drug trafficking, as Colombia remains the largest producer of cocaine in the world.
On this point, Mr. Petro proposes to rethink the failure of the crop eradication policy, in collaboration with the United States, the main consumer of this coca leaf derivative.
Mr. Petro will offer peace to these armed groups in exchange for sentence reduction programs, on the model of the agreement with the FARC.
He also intends to reform the riot police, accused of multiple human rights violations during the violent repression of the demonstrations which shook the mandate of his predecessor.