RIO DE JANEIRO -- Criminal groups set at least 35 buses on fire in the Brazilian city of Rio de Janeiro on Monday, according to the industry group that represents bus companies, after police killed a crime boss in an operation.

The attacks on buses were concentrated in the west of the city, where rival criminal groups are fighting for territorial control. They came after a police operation that killed the nephew of the leader of the state's largest militia, according to the police.

Rio's so-called militias, often composed of current and former police officers, have become one of the region's largest security threats. Originally set up as self-defense forces for poor neighborhoods blighted by drug gangs, they have now metastasized into criminal outfits operating in multiple different rackets.

Industry group Rio Onibus said 20 of the vehicles were city buses, five were part of the city's rapid transit fleet and the remainder were chartered or tourism buses.

Rio de Janeiro state Governor Claudio Castro said the operation was a "heavy blow" to the militia.

"In addition to being related to the criminal (head of the militia), he acted as the paramilitary group's 'man of war,' being the main person responsible for the turf wars that terrorize residents in Rio," Castro said in a social media post.

The attacks disrupted the city's bus rapid transit system, hindering the commute home for residents and workers in the region. Classes in public schools in the area were also canceled.

The Military Police said it arrested 12 suspects involved in the fires, in addition to preventing 15 individuals from setting a cargo truck on fire in one of the city's main gateways.