Civil defense officials said the explosion occurred at 5:00 am local time and may have resulted from a leak in a gas line.
Residents reportedly complained of a gas smell for more than a year.
Images on Globo News television showed a blown manhole cover in the sidewalk outside the building, and shattered glass.
The injured were rushed to area hospitals.
The building where the explosion occurred is in Fazenda Botafogo, a complex of 86 apartment buildings housing some 17,000 people near a main access road leading to the center of Rio.
"We are going to look into the complaints of residents, who said there have been odors of gas for more than a year," said Cristiane Delart, a CEG spokeswoman, told the G1 news website.
Delart said company technicians had carried out inspections in response to the complaints but had found no problems.
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