Nine inmates were killed and 20 others injured in a fight between rival gangs at a prison in the northern Brazilian city of São Luis, authorities said Thursday.

The clash at Pedrinhas penitentiary involved jailed members of two drug-trafficking outfits, the Maranhão state government said.

Police recently arrested 16 reputed members of one of the two gangs, Bonde dos 40, which is seen as the most powerful criminal organization in the state.

The brawl inside the prison was followed by the burning of eight municipal buses in attacks across the city of São Luís. Police suspect the arson incidents were connected to the conflict at Pedrinhas. EFE