Toronto fire crews spent Sunday afternoon battling a five-alarm blaze in the city's east end that broke out after an underground hydro vault directly in front of a highrise building exploded.

Fire officials say the force of the explosion caused "significant damage" to the building, blowing out doors and moving parked cars. One fire truck was lifted off the ground before dropping back down from the force of the blast.

A structural engineer was on the scene to assess the damage.

More than 40 fire trucks were at the 21-storey building on Secord Avenue, in the Danforth Avenue and Dawes Road area. Emergency crews got a call at about 10:43 a.m. about smoke in the underground of the building.

When crews arrived, the area was clear, Fire Chief Bill Stewart told reporters at the scene. But then, something blew and there was a huge ball of fire, shooting about 20 to 30 in the air.

Richard Grant, a resident who lives in an apartment building across the street, told CTV.ca he heard the explosion.

"I heard a big boom but it was quite overcast so I just thought it was a lightning bolt," he said. "Then I saw the smoke in the air."

One woman, who said she is hearing impaired, also felt the blast.

"I am fully deaf but it was so loud, I could hear the explosion," Hailey Hudson told CTV Toronto.

Six firefighters have been injured, three of them with second-degree burns. Three firefighters and a resident of the building were also taken to hospital for smoke inhalation. None of the injuries are considered life-threatening.

At first, fire crews decided it would be safer to let residents stay in their homes because of the heavy smoke in the area. However, now that the smoke has cleared, Stewart said it will take emergency crews a while to evacuate the residents because many of them are old and have to be carried down the stairs.

"The elevators aren't working and there is no power in the building," he said. "If they return tonight is dependent on the situation. They have no hydro, no water. If that continues, we have to look at secondary means for shelter for those individuals."

Residents living in a nearby townhouse complex were evacuated to a nearby school as a precautionary measure but they have since been allowed to return to their homes after officials confirmed there were no toxic PCBs released into the air.

Some adjacent buildings were damaged because of the blast.

"We heard an explosion and our building shook," said one resident. "I've got broken windows. I've got pictures and stuff laying on the floor."

About six blocks in area has been closed to traffic and motorists are warned to take alternate routes.

With a report from CTV Toronto's Dana Levenson