Police and protesters violently clashed in Rio de Janeiro's main train station in a demonstration against a 10-cent hike in bus fare.
A cameraman for Band TV was hit in the head by either a stun grenade launched by police or a homemade explosive tossed by protesters; it was not immediately clear which.
Band said in a statement that cameraman Santiago Andrade was taken to a hospital by police and underwent surgery. He is in serious condition.
It is the latest protest to hit Brazil since last June, when nationwide demonstrations broke out after a sharp police crackdown on a group in Sao Paulo that was marching against an increase in public transportation fares.
That increase was reversed in the face of protesters' pressure.
Rio Mayor Eduardo Paes recently approved a 10-cent increase for bus fares starting this Saturday.
About 800 protesters had peacefully gathered on Thursday in central Rio before they started marching to the city's main train station, some holding aloft signs condemning the billions of dollars spent to host the World Cup, money they want used for better hospitals, schools and infrastructure.
Clashes broke out inside the train station after demonstrators began jumping over turnstiles and police used batons and tear gas to disperse members of the Black Block anarchist tactic.
Police pushed the demonstrators outside and used more tear gas to disperse those gathered, while demonstrators hurled rocks at the officers.
Authorities were forced to close the station, leaving thousands of commuters stranded. Some bystanders were made ill by the tear gas, while others fainted.
Thais Jorao, a 22-year-old protester, said that demonstration was not simply because of the 10-cent bus fare hike.
"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."