Police used tear gas and batons to disperse opponents of a bus fare hike in Rio late on Thursday. The protest had begun peacefully at Rio's main rail station.
The television channel Band TV said its cameraman Santiago Andrade was in serious condition after being hit in the head either by a stun grenade launched by police or a homemade explosive.
The 10 percent fare rise, due to come into effect on Saturday, stems from a recent decision by Rio's mayor Eduardo Paes. A similar rise last June was later withdrawn after mass protests.
An Associated Press correspondent said 800 protestors had marched peacefully to the station, some holding placards condemning billions being spent to host this summer's World Cup instead of boosting social services.
Inside the station, some demonstrators jumped turnstiles. Police pushed the protestors outside and used tear gas to disperse them. Authorities then closed the station, leaving thousands of commuters stranded.
DW's Philipp Barth was on the scene for the protests. He reported the Brazilian police hit his camera out of his hands. Barth said authorities "completely overreacted" and were totally unprepared for demonstrations of such magnitude.
Protestors demand social improvements
One protestor, Thais Jorao, said the protest was for various reasons.
"If it was a public transportation fare hike when we had good health services and education, you wouldn't have this many people on the street," he said. "On top of this you see spending with the World Cup, things that we really don't need. We want health, education, decent public transportation."
Cup security checked
Andrei Rodrigues, Brazil's special secretary for security and safety at major events, said at least 100,000 security personnel would be deployed across Brazil for the World Cup - more than double than for the 2013 Confederations Cup.
Rodrigues spoke during a week-long tour of the 12 cities that will host matches in the 32-team World Cup, the biggest sporting event ever staged by Brazil.
The month-long World Cup kicks off on June 12.
ipj/msh (AFP, dpa, AP)