Brazilian riot police fired tear gas, stun grenades and rubber bullets to disperse extremist protesters who ransacked bank branches, shops and a car dealership in central Sao Paulo, AFP reports.
A police spokesman told AFP that a total of 20 people were arrested during the clashes, which followed a peaceful march by 300 demonstrators against Sao Paulo state Governor Geraldo Alckmin.
The protest, called on social media by the Black Bloc anarchist group, also demanded the demilitarization of the state police, who have been accused of using excessive force in previous disturbances.
Riot police fought running battles with a small group of extremists, who, armed with hammers, went on a rampage in the Pinheiros neighborhood, smashing windows in banks, shops and other businesses.
The police hurled tear gas, rubber bullets and stun grenades against the vandals, some of them masked, who set fire to piles of trash and sprayed graffiti on walls and buildings.
There was no official word on casualties, but an AFP photographer saw one demonstrator injured by flying glass.
Police, who earlier warned that they would use force to stop "criminal acts," tightened security to prevent the bulk of the demonstrators from reaching the city's main Avenue Paulista, where 50 demonstrators chanted anti-Alckmin slogans.
"We want Alckmin to go. He is a corrupt capitalist," said 25-year-old Orlando Azevedo.
Various police units were deployed on the Avenue Paulista, Sao Paulo's financial heart, where last Friday some 300 extremists wrecked around 10 banks and set fire to a television news van.
Friday's violence erupted after a march in support Rio de Janeiro demonstrations against state Governor Sergio Cabral, who is accused of corruption.
In Rio last week, around 200 protesters marched to within a few meters (yards) of a Copacabana beach stage where visiting Pope Francis was wrapping up a massive ceremony with hundreds of thousands of young Catholics.
Last month, Brazil was jolted by a wave of massive street protests in scores of cities to demand better public services and an end to endemic corruption.
The unrest began in Sao Paulo with young militants demanding a rollback of public transport fare hikes, a call picked up in several other major cities. The fare increases were subsequently canceled.