Gustavo Petro, a former urban guerrilla who was jailed for his political beliefs in the 1980s, was sworn in as Colombia’s president on Sunday at the head of what is likely to be the most leftwing government in the country’s history.

In a ceremony rich in symbolism, Petro, 62, took the presidential oath in Bogotá’s Plaza de Bolívar, outside Congress and opposite the Palace of Justice. In 1985, while Petro was in prison, the urban guerrilla group he belonged to stormed the palace. About 100 people were killed as the army tried to take it back.

In his speech, Petro repeated many of the promises from his election campaign, saying that after decades of bloody conflict Colombia needed a complete overhaul to make it “a potential for life”.

“We are here against all odds . . . against those who did not want to let go of power. But we did it,” he said.

Petro’s history-making running mate, Francia Márquez, was also sworn in on Sunday as Colombia’s first black vice-president. Born into poverty in the violence-plagued south-west of the country, she is an environmental activist and won the prestigious Goldman Prize in 2018.

In a ceremony that broke with tradition, butterfly motifs in Colombia’s national colours — yellow, blue and red — were projected on screens flanking the stage. Unlike during previous government handovers, thousands of people were allowed into the square.

Across the city, Petro supporters gathered in parks and squares to watch the ceremony on big screens. Some waved the red, white and blue flags of M-19, Petro’s former urban guerrilla group. “It’s a great victory for the Colombian people to finally have a people’s government, far from the oligarchy and political clans,” said Xiomara Jimenez, a psychologist, as a drum circle played and street vendors hawked flags to revellers. “I’m happy for my children and for my people.” Jairo Barahona, a barber from Bogotá, said he expected Petro to help the working and middle classes. “There are opportunities now to support small businesses and workers,” he said.

On the campaign trail, Petro promised wholesale land reform, a halt on oil and gas exploration and to fund universal access to healthcare and higher education. He also pledged to tackle corruption in the military and public life and to implement the 2016 peace deal with the Revolutionary Armed Forces of Colombia (Farc), which stalled during the administration of Iván Duque. Petro has also signalled a willingness to negotiate with the National Liberation Army (ELN), another leftist guerrilla group. A new footing in the drug war is also likely. Colombia is the world’s top producer of cocaine, and the drug continues to fund armed groups across the country. “It is time for a new international convention that accepts that the war on drugs has failed,” Petro said in his inauguration speech. “The war on drugs has strengthened mafias and weakened states.”

Passing tax reform will be among Petro’s foremost legislative priorities if he is to fulfil campaign pledges. Failing to do so would risk alienating his base, which is partly made up of disaffected young people who marched in the thousands last year in protest against Colombia’s inequalities. Petro’s finance minister, José Antonio Ocampo, a market-friendly, Yale-educated economist, has said that a tax reform bill will be sent to congress on Monday. Before taking the oath, Petro held meetings with presidents from across the region, including Alberto Fernández of Argentina, Luis Arce of Bolivia and Guillermo Lasso of Ecuador. Peru’s embattled president, Pedro Castillo — who is facing numerous judicial investigations — was barred by his congress from travelling to Colombia.

The US delegation was led by Samantha Power, head of development agency USAID. Colombia has traditionally been Washington’s staunchest ally in the region, although the relationship could be tested under a Petro government. Nicolás Maduro, Venezuela’s strongman, was not invited despite Caracas and Bogotá being poised to reset their frosty relationship. “Petro is likely to present his priority reforms, which are tax, agricultural and anti-corruption policy in the first few weeks,” said Silvana Amaya, a Bogotá-based political risk analyst with Control Risks. “This would let us know how strong the relationship between Petro and legislative is going to be, marking his entire reform agenda in congress.”