The abandoned building, 26 storeys high, was occupied by squatters and firefighters warned the death toll could rise.
Flames were said to have taken hold on the fourth or sixth floor at about 1.30am local time on Tuesday.
The building, a former headquarters of the federal police, collapsed about less than 90 minutes later.
According to one resident of a neighbouring building, the “majority” of a reported 150 people were evacuated before the collapse.
Romulo de Souza, 49, added that neighbours believed the fire may have been started by a gas leak.
Firefighter Lieutenant Andre Elias told Globo TV that at least one person had been killed in the collapse. Authorities were working to locate several others who were missing.
Chunks of flaming debris crashed into nearby buildings as the tower collapsed. Video showed dust, flames and sparks flying into the air.
Firefighters believe clearing the rubble, which was still smouldering hours afterward, could take days.
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