This morning's arrests are apparently aimed at preventing more protests during Sunday's final World Cup match between Argentina and Germany in Rio's Maracana stadium.
Police tell the G1 news portal says police seized gas masks, fireworks and firearms.
Calls to the police department for more details went unanswered.
Yesterday, police said more than 25,000 soldiers and police will be deployed in the city on Sunday. It's the biggest security detail in Brazil's history.