Brazil's far-right presidential hopeful, Jair Bolsonaro, officially kicked off his campaign on Sunday, excoriating the political center but softening his incendiary stance on gays and blacks three months before wide-open October elections.

The 63-year-old former army captain, who leads in polls that exclude jailed former leftist President Luis Inacio Lula de Silva, made his announcement to an elated crowd a day after centrist parties threw their support behind centrist rival Geraldo Alckmin.

"Once again, thank you Geraldo Alkmin for uniting the scum of Brazilian politics," Bolsonaro said, referencing the four-time governor of Sao Paulo, candidate for the Brazilian Social Democracy Party, who trails Bolsonaro by 10 points or more.

Some 3,000 rowdy supporters, including one impersonator of US President Donald Trump, gathered in the crime-ridden city Rio de Janeiro to welcome his candidacy, chanting Bolsonaro slogans and making pistol gestures with their hands.

Bolsonaro's anti-corruption message and vows to loosen gun controls to fight crime resonate deeply in the city, where a crime wave has prompted the federal government to intervene in the state's security. A massive corruption scandal at Petrobras that has saddled the state-controlled oil giant with billions in debt has also weighed on the city.