The force of the blast at this Kingston Road-Eglinton Avenue building sent a metal door flying into a parking lot 50 metres away. (CBC)

It could be days before about 500 residents of an east-end Toronto highrise condominium building are able to return home after an explosion early Thursday knocked out power and forced an evacuation, the city's utility said.

The explosion occurred around 2:30 a.m. in the building's basement electrical room at Kingston Road and Eglinton Avenue, with the force of the blast sending a small metal door flying into a parking lot more than 50 metres away.

The blast was quickly followed by a Toronto Hydro transformer exploding outside the building, fire officials said.

The Ontario fire marshal is investigating what caused the explosion and needs to give the utility clearance before repair crews can enter the building.

Engineers were checking whether the building has structural damage and Toronto Hydro spokeswoman Tanya Bruckmueller told CBC News the utility doesn't know how long repairs will take.

'Really big bang'

Residents, some dressed only in pyjamas and bathrobes, rushed from the building as smoke emerged from the basement. Some said their lights flickered and the building's alarm system sounded just before the blast.

Gail Watson said she was sleeping in her third-floor condo when she heard a bang. She looked out the window and saw smoke, then she and her daughter-in-law heard "another really big bang."

"We knew it was serious, so we wanted to check it out," Watson told CBC News. "We came out to see what was going on and then we couldn't get back up."

But all night she worried about her son and two young grandsons, aged one and two, who were still in the apartment without electricity. The children finally left the apartment at around 10 a.m.

Most residents were able to enter their condos to take some of their belongings and pets with them. Some spent the day at a nearby community centre, while others said they were going to stay with friends or family.

Toronto Fire Capt. Adrian Ratushniak told CBC News the incident is not as serious as another explosion last summer in the Danforth neighbourhood, which displaced residents for more than a month.