Police fired tear gas and arrested scores of protesters in Sao Paulo late Friday when a march demanding free public transportation for students turned violent.
A group of vandals, some wearing black hoods, smashed store windows and bank ATM machines, set a public bus on fire, and destroyed subway and bus ticket machines.
Riot police wielding clubs and shields in turn fired tear gas to disperse the mob. Sao Paulo police blamed the anarchist group "Black Bloc" for the violence.
At least 78 people were arrested in the protest, police told AFP.
The protest briefly blocked key city streets and disrupted the public transportation system in the city of 11 million.
A group that police described as "criminals disguised as demonstrators" even surrounded and attacked a police colonel.
"Since the start of the demonstration the presence of Black Bloc members, who screamed at police and tried to get them to react violently, was noticed," city police told AFP in a statement.
The anarchists "attacked Colonel Reynaldo Simoes Rossi in a cowardly fashion, stealing his pistol and his radio. The officer had his collar bone broken and multiple lacerations to his face and head," the report read.
Broadcast images showed a sole plainclothes officer waving his pistol in an attempt to guard the colonel, who was on the ground being attacked by the masked thugs.
Brazil is in the international spotlight as it prepares to host the World Cup next year and the Summer Olympics in 2016.
More than a million Brazilians took to the streets in June, angry at widespread corruption and the millions being spent on the sporting events. Protesters say the money should instead be invested in improved transportation, education and health services.
In a June protest in Sao Paulo, tens of thousands marched over the poor condition of the public transportation system.
Smaller protests have since been taking place in Sao Paulo and Rio de Janeiro. Those protests are often led by radical groups and usually end up in clashes with police.
Black Bloc members wear black hoods, T-shirts or scarves to cover their faces. They move in groups, and often break windows and attack police at otherwise peaceful demonstrations.