Gustavo Petro, a former fighter of the now demobilized M-19 rebel group, has been sworn in as Colombia’s first leftist president.
Petro, 62, took his oath of office on Sunday, vowing to unite the polarized nation, make peace with rebel groups and criminal narcotic gangs, while fighting against climate change.
"I swear to God and promise the people that I will faithfully enforce the Constitution and the laws of Colombia," Petro said in his oath of office.
Petro, a former mayor of capital Bogota and current senator, has pledged to fight inequality with free university education, pension reforms and high taxes on unproductive land.
"I do not want two countries, just as I do not want two societies. I want a strong, just and united Colombia,” Petro pointed out in his inaugural speech.
Petro condemned the decades-long US-led fight against narcotics, saying, "It is time for a new international convention recognizing that the war on drugs has failed."
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.
During the election campaign, Petro vowed to 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.
He called for the establishment of an international fund to help protect the Colombian Amazon from deforestation.
Petro has also pledged to fully implement a 2016 peace deal with FARC rebels and seek talks with the still-active ELN guerrillas.
At least nine Latin American presidents, the Spanish monarch, and about 100,000 guests attended the inauguration ceremony which was held at Bogota’s Bolivar Plaza.
Thousands of Petro's supporters celebrated in Bogota, and at large screens set up in public places across the country.