As night fell on the party that drew hundreds of thousands of people, police and municipal guards sought to disperse crowds with people fleeing along the beach and waterfront. Guards chased a man, tackling and hitting him before handcuffing him. Some people threw bottles.
It was unclear how the disorder began.
"We were peacefully enjoying the carnival when police came throwing bombs," said Isaque Batista, a 22-year-old barber. "There was no previous tumult. The police arrived throwing (tear gas) bombs when there was no need for it."
Authorities said security forces dispersed crowds after a group of municipal guards was attacked with glass bottles, stones and other objects. There was no report of how many people were detained. A municipal guard suffered minor injuries.
Military police had originally objected to holding the party in Copacabana as they weren't provided enough time to organize security for the event.