Gustavo Petro, a former rebel fighter, has been sworn in as Colombia’s first left-wing president.

The former member of the M-19 armed group took his oath of office in Bogota’s Bolivar Plaza on Sunday afternoon, pledging to unite the polarised country in the fight against inequality and climate change, and achieve peace with rebel groups and crime gangs.

His inauguration was watched by some 100,000 invitees including Spanish King Felipe VI, at least nine Latin American presidents and other Colombians invited by Petro.

"I do not want two countries, just as I do not want two societies. I want a strong, just and united Colombia,” an emotional Petro said in his inaugural speech. "The challenges and tests that we have as a nation demand a period of unity and basic consensus.”

Petro, a 62-year-old former senator, has promised to revive scuppered peace negotiations with the National Liberation Army (ELN) rebels and to apply a 2016 peace deal to ex-members of the FARC fighters who reject it.

His foreign minister has said the government will hold dialogue with gangs and potentially give members reduced sentences in exchange for information about drug trafficking.

Armed groups should accept that deal, Petro said.

"We call on all those who are armed to leave their arms in the haze of the past. To accept legal benefits in exchange for peace, in exchange for the definitive non-repetition of violence,” Petro told the cheering crowd gathered under a bright Andean sun.

He also called for a new international strategy to fight drug trafficking, saying the United States-led war on drugs has failed.

"It is time for a new international convention that accepts that the war on drugs has failed, that it has left a million dead Latin Americans during 40 years and that it leaves 70,000 North Americans dead by overdose each year. The war on drugs strengthened mafias and weakened states,” he said.

Climate change must be fought internationally, but especially by countries which emit the most greenhouse gases, Petro added, saying Colombia would transition to an economy without coal or oil.

On the campaign trail, Petro said Colombia will stop granting new licenses for oil exploration and will ban fracking projects, even though the oil industry makes up almost 50 percent of the nation’s legal exports. He plans to finance social spending with a $10bn-a-year tax reform that would boost taxes on the rich and do away with corporate tax breaks.

Petro has also said a top priority is to fight hunger in the country of 50 million, where nearly half the population lives at some level of poverty. He has also pledged free public university education and healthcare changes, and constructed a broad congressional coalition of left-wing and centrist parties to pass his platform.

The new president starts from an "enviable position, with a large majority in Congress and, in terms of the street, with support that no government had in recent years,” analyst Jorge Restrepo of the Resource Center for Conflict Analysis, told the AFP news agency.

But others said Petro must prioritise his policy goals.

"He’s got a very ambitious agenda,” said Yan Basset, a political scientist at Bogota’s Rosario University. "But he will have to prioritize. The risk Petro faces is that he goes after too many reforms at once and gets nothing” through Colombia’s congress.

New Vice President Francia Marquez, an environmental activist and former housekeeper, is the first Afro-Colombian woman to hold her post. Their cabinet includes an internationally renowned professor, Jose Antonio Ocampo, as finance minister and an academic who researches the negative impacts of industries as minister for mining. The labour ministry will be led by the head of Colombia’s communist party.

Thousands of supporters celebrated in Bogota and at large screens set up in public places around the country.

"I didn’t believe I would live to see this finally happen,” said Nelson Molina, a 56-year-old plumber who was sporting a Petro t-shirt and hat. "I know we won’t change from one day to the next, this is just the beginning.”

Groups of people were also celebrating on both sides of the Colombia-Venezuela border, with dozens gathered on either side of a crossing point on the Simon Bolivar bridge outside the Colombian city of Cucuta.

Petro, a former mayor of Bogota, has promised to reopen diplomatic relations with Venezuela, allowing trade between the two countries and consular services to resume.

Petro’s first order as president was for the military to bring the sword of Latin American liberation hero Simon Bolivar — stolen by Petro’s erstwhile M-19 comrades in 1974 — to be displayed in the plaza, after his predecessor Ivan Duque did not authorise its use in the ceremony.