Hundreds of protesters have attacked a bus station in the Brazilian city of Sao Paulo, setting fire to a bus and destroying cash and ticket machines.
Riot police responded with tear gas and at least six people were arrested.
Protesters were demanding free public transport. High transport costs triggered protests in Sao Paulo in June that later spread across Brazil.
The clashes came as President Dilma Rousseff announced a plan to improve public transport in the city.
Friday's protests began with a march through the city that lasted about three hours.
Violence erupted when a section of the crowd broke off and attacked a bus terminal in the city centre, police said.
Masked protesters, said to include members of the so-called Black Bloc anarchist group, set light to a bus and vandalised cash machines and ticket turnstiles.
At one point a police colonel was struck by a rock and had to be rescued by fellow officers, Brazil's O Globo newspaper reported.
As police moved in, protesters fled, setting up barricades in some parts of the city centre.
On Friday, President Rousseff announced a $2.4bn (£1.5bn) plan to improve public transport in Sao Paulo.
The money will be invested in urban train lines and subway expansion including a new line to link the city's airport to Sao Paulo's eastern zone.
"The republic and the state government have a common co-ordinated action to attack the most serious problems that we have in our largest cities, one of which is urban mobility," said Ms Rousseff.
Although June's protests were triggered by transport costs, wider unrest has been targeted at Brazil's rising cost of living, poor public services, corruption and the misuse of government money.
Earlier this month, a march by thousands of teachers in Rio de Janeiro demanding a pay rise was hijacked by anarchists who set fire to banks and other buildings.
Continuing street protests are causing concern ahead of the 2014 World Cup that Brazil will host in a few months time.