The chaotic scenes erupted on Rio’s west side after an operation that Rio state governor Claudio Castro said killed the nephew and right-hand man of a militia boss known as “Zinho,” the group’s alleged leader in the region.

Rio’s mayor Eduardo Paes called the militia members “idiots as well as criminals,” in a post on X, formerly known as Twitter.

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

City hall declared a level-three alert on its five-level scale over the unrest in the popular tourist destination, warning residents of “high-impact incidents.”

Public bus operators’ association Rio Onibus said 35 buses had been set on fire, including five used on the special rapid-transit lines launched for the Rio Olympics in 2016.

Public transport was partly suspended on the city’s west side.

Police operations against heavily armed criminal gangs are common in Rio, where poor favela neighbourhoods are often caught in the crossfire.

Militia groups control more than half the city’s territory, installing a reign of terror in poor neighbourhoods that are home to more than two million people, according to a 2020 study by a consortium of universities, online watchdog platforms, and a government anti-crime hotline.

Militia members are often former police officers.

They initially formed as neighbourhood watch groups to protect residents from drug gangs in the city, which is known for its picturesque beaches but also its violent crime.

The militias, however, soon evolved into organised crime groups themselves, controlling sectors including internet service, cable television, transportation, and construction.

They have been involved in running turf wars with drug gangs on the city’s west side.

The latest violence comes after three doctors visiting Rio for a conference were gunned down at a beachside bar in an upscale neighbourhood three weeks ago.

Investigators said they appeared to have been targeted by mistake by criminals who mistook one doctor for a militia member.