Witnesses said some people were unable to get off the buses in time and at least 10 people were reportedly injured in the fires.
Gang members also attacked police stations and posts, killing two officers, according to O Globo.
Most of the attacks occurred on the city's poor north side. But at least 12 shots were reported fired on a police post in the upscale Lagoa district.
Rio de Janeiro is one of the most violent cities in the world, with an annual murder rate of around 50 per 100,000 residents.