The cause of the collapse remains unclear, but witnesses spoke of an explosion and a strong smell of gas.
City Mayor Eduardo Paes said that they were focusing on rescue efforts before looking into the incident's cause.
According to the BBC's Paulo Cabral, rescue workers were able to pull a cleaner from inside one of the elevators in the rubble after he managed to call a friend on his mobile phone.
The buildings - located near the Municipal Theatre and the headquarters of oil giant Petrobras - crushed a four-storey construction site on their way down.
The area surrounding the buildings is now covered in rubble, with several cars partially covered by debris.
Dozens of emergency workers are at the scene and police have cordoned off the area, our correspondent adds.
'Like an earthquake' Electricity to the street has been cut off for safety reasons.
One witness, who identified himself as Gilbert, told Reuters news agency: "It was like an earthquake. First some pieces of the buildings started to fall down. People started to run. And then it all fell down at once."
The incident comes a little over three months after a suspected gas explosion at a restaurant in the city left three people dead.
Concerns have been raised about the state of Rio de Janeiro's infrastructure as Brazil prepares to host football's World Cup in 2014 and the Olympic Games two years later.