Emergency services arriving in the working-class district of Pechatniki helped 152 injured people, 73 of whom went to hospital. However, the outbreak of a fire after the blast hampered further rescue work and it was feared that dozens more victims might be trapped under the smoking rubble.
The blast at midnight on Wednesday punched a gaping hole in the building at 19 Guryanovo Street, bringing down two staircases and 54 flats, in which 202 people lived. Some victims were found at number 17 while windows were shattered in 15 other buildings. The explosion was like an earthquake, said a survivor.
President Boris Yeltsin expressed shock and sympathy.
Because of old pipes, gas explosions are common in Russia. That was the first theory put forward to explain the blast. However, Mr Luzhkov said that experts had ruled out a problem with the main gas supply.
That left the possibility that gas canisters, sometimes stored by Russians for use at country cottages, had exploded or that terrorists were to blame, he said. He did not rule out the involvement of Islamic rebels fighting Russian rule in the southern region of Dagestan.
Residents said a few minutes before the blast four men were seen speeding away from the building in a car. Interfax news agency said a man speaking with a Caucasian accent had telephoned and appeared to claim responsibility. "What happened in Buinaksk and Moscow is our answer to the bombing of Dagestan," he said.
At the weekend, a bomb wrecked flats occupied by Russian army families in the town of Buinaksk, Dagestan.