Suspected members of Brazilian criminal groups set fire to at least 35 buses in Rio de Janeiro on Monday, after one of their leaders was killed in a police operation, the Deutsche Welle (DW) reports citing officials.

The attacks were concentrated in the west of the city, where so-called militias are fighting for control.

Rio Mayor Eduardo Paes said those responsible were "idiots as well as criminals" in a post on social media.

"Militia members on the west side are burning public buses paid for with public money to protest against a police operation," he wrote.

Transport group Rio Onibus said 20 of the vehicles were regular city buses, five were part of a rapid transit line built for the 2016 Olympics, and the remainder were tourist coaches.

The incident disrupted public transport in the area, causing headaches for commuters and leading classes at some schools to be canceled.

The military police said it arrested 12 people for suspected "terrorist actions."

City hall also declared a level-three alert on its five-level scale, warning residents and tourists of possible "high-impact incidents."

The bus burnings occurred shortly after a police operation killed the nephew and right-hand-man of a militia boss known as "Zinho."

Rio de Janeiro state Governor Claudio Castro said the operation was a "heavy blow" to his 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 on Monday night.

Rio's so-called militias are often made up of current and former police officers.

They were formed as neighborhood protection outfits to combat drug gangs but have since turned into criminal outfits themselves controlling more than half the city's territory and running multiple different rackets.