A blaze that kept firefighters on the scene well into the afternoon on Saturday left the Glebe apartment building it devastated so unstable that it will have to be torn down, leaving 10 tenants without homes.

The fire at 5 Monk St. broke out just before 7 a.m. Saturday, escalating to a three-alarm fire in just over 20 minutes. Twenty trucks and 55 firefighters worked to put out the blaze at its peak. Bank Street, Monk Street and Fifth Avenue were all closed down for several hours after the blaze.

The fire started in the third floor near the attic and worked its way down, ripping quickly through the old-style construction building, made of plaster and lath. There's at least $750,000 in building damage and $250,000 in contents damage.

Officials are unsure if there were working smoke alarms in the building; the Ottawa police are in the middle of a campaign to raise awareness about the issue, and remind residents to double-check the batteries regularly.

Pedestrian passer-by injured

Although no injuries were reported to firefighters and residents, a passer-by on Bank Street slipped on some ice created from water flowing underneath one of the hydrants. The person was taken to hospital with undisclosed injuries.

People living in a four-unit apartment building next door to the fire were also evacuated since the power is shut off and there is water damage to the building. There is no word yet on when they can return.

The Red Cross came almost immediately to help the displaced residents, said public information officer Pierre Savary, but most already had friends or family they could stay with.

Due to the severity of the fire, Savary says it's likely the fire marshall's office will be called in to investigate.

With a report from CTV Ottawa's Kate Eggins