The plane went down in the busy Barra Funda neighbourhood on the city’s west side, near its downtown.
A piece of the plane hit a bus, injuring one woman inside, while a motorcyclist was struck by another piece of wreckage, the firefighters’ statement said. Both were receiving medical care.
A later statement said that four additional victims at the site were taken to hospitals in the area with minor injuries. Officials had no immediate information on the identities of the two fatalities.
A CCTV video obtained by the local television network TV Globo showed the moment of the crash.
The plane slammed into the avenue a few metres past an intersection, where a line of cars was waiting to move forward.
Images on local media showed the plane’s fuselage and the bus on fire, with firefighters working to extinguish the blaze.
The avenue is home to office buildings and there is a key bus, train and subway station nearby.
There was no word as to what had caused the plane to lose altitude and smash into the avenue less than 5 kilometres from the private airport from which it had departed.
It was bound to Porto Alegre, in the southern Rio Grande do Sul state.