Petro, a former member of the M-19 guerrilla group, was sworn in by Senate President Roy Barreras in Bogota's Bolivar Plaza on Sunday afternoon, 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 inauguration speech on Sunday.Â
"The challenges and tests that we have as a nation demand a period of unity and basic consensus."
Petro has promised to revive scuppered peace negotiations with the National Liberation Army (ELN) rebels and apply a 2016 peace deal to ex-members of the FARC guerrillas 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.
He also called for a new international strategy to fight drug trafficking., saying the US-led war on drugs had failed.
"The war on drugs strengthened mafias and weakened states," Petro said.
Climate change must be fought internationally, but especially by countries that emit the most greenhouse gases, Petro said, adding Colombia would transition to an economy without coal or oil.
A $US5.8 billion ($A8.4 billion) tax reform, which would raise duties on high earners to fund social programs, will be proposed to congress on Monday by new Finance Minister Jose Antonio Ocampo.
Petro, a 62-year-old former senator, has also said a top priority is to fight hunger in the country of 50 million, where nearly half the population lives in poverty.
Promises of pension reform and a halt to new oil development have caused investor jitters despite the appointment of Ocampo, a long-time official.
New Vice-President Francia Marquez, an environmental activist and former housekeeper, is the first Afro-Colombian woman to hold her post.
