Two Argentine fans and a Brazilian were arrested after the brawl, the state's Military Police said, ahead of the final on Saturday in South America’s equivalent of Europe's Champions League.

"CONMEBOL calls on the fans of Boca Juniors and Fluminense to share together the moments of joy and celebration that our football gives us," the organisation said on its official X account.

"The values of the sport we are most passionate about should inspire peaceful and harmonious behaviour. We therefore repudiate any acts of violence and racism that may occur in the context of the final."

Police said local security agents intervened after seeing fans clash on the Copacabana beach fan fest.

"Officers immediately surrounded the area, brought the situation under control, and two men - Argentine tourists and a third man, Brazilian - were taken to the 12th Police Station," police said in a statement.

Locals said Argentine visitors were responsible for the violence, while Boca fans told Argentine media that Fluminense supporters arrived at the beach to carry out an organised attack before police fired tear gas and rubber bullets.

Some 20,000 Boca fans are expected to attend the final at Maracana stadium on Saturday as their team seeks its seventh Libertadores title, said Argentine news outlet La Nacion.

Fluminense, playing on home ground, hope to lift their first continental trophy, after losing to Ecuador's Liga de Quito in the 2008 final.

(Reporting by Janina Nuno Rios in Mexico City; Editing by William Mallard)