Suspected paramilitary militia members torched 35 buses and a train drivers' cabin in Rio de Janeiro Monday, paralyzing parts of the scenic Brazilian city after one of their leaders was killed in a police operation, officials said.

The chaotic scenes erupted after an operation that state Governor Claudio Castro said killed the nephew and right-hand-man of a militia boss known as "Zinho."

Castro said the police had delivered a "hard blow to one of the largest militias in the west" of Rio, describing the nephew, who goes by the alias "Faustao" as a "known warlord."

Police subsequently arrested 12 people for suspected "terrorist actions" and city hall declared a level-three alert on its five-level scale, warning residents and tourists of possible "high-impact incidents."

Rio Mayor Eduardo Paes called the militia members "idiots as well as criminals," in a post on X, formerly Twitter.

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

Public bus operators' association Rio Onibus said 35 buses had been set on fire — the most in a single day in the city's history.

They included five buses used on special rapid-transit lines launched for the Rio Olympics in 2016.

The Supervia train operators' organization said a train cab was also set on fire after the driver was ordered out.

Public transport in the city was partly suspended.