Witnesses say they saw large pieces of burning wood flying across the street. (CBC)

A 50-year-old man was in police custody Wednesday morning in connection with an enormous fire that destroyed several buildings in Montreal's Plateau district Tuesday night.

Witnesses said they saw a man wandering around the building just before the fire started.

Police picked up a man late Tuesday night and held him overnight for questioning.

The fire started at about 9 p.m. on Tuesday in a three-storey condominium building under construction, near the intersection of St-Andre and Roy streets. It quickly spread to six neighbouring buildings, four of which were destroyed.

About 150 firefighters fought the flames; two were treated for heat exhaustion, and another suffered a hand injury. (CBC)

The cause of the fire is not yet known, but fire officials are calling it suspicious and have handed the case over to Montreal police.

Witnesses said they saw large pieces of burning wood flying across the street. The fire spread so quickly some tenants in neighbouring buildings were forced to leave their homes in pyjamas. About 100 people in total were evacuated from their homes.

On Wednesday morning, the street corner was filled with billowing smoke. The area surrounding the fire site remained blocked to traffic Wednesday morning, in a zone marked by Napoleon, De Mentana, Cherrier and Saint-Hubert streets.

About 150 firefighters battled the fire; two were treated for heat exhaustion and another suffered a hand injury.

Eyewitnesses describe blaze as 'amazing'

Yan Visell, who lives about 100 metres from the building, said his apartment was not affected but he had to flee quickly because the fire spread so quickly.

"You just had heat and a lot of smoke coming in the window. And I could see the flames over all the buildings and into the street and then a shower of sparks coming across the street," said Visell.

Pierre Joyal said he called 911 as soon as he saw the flames.

"I had time to call 911 and when I put my head up again everything was on fire. It went so fast it's amazing," said Joyal.

Marie-Lou Carrier said she went into the neighbouring building to help an elderly woman out of her apartment.

"When I was on the third floor, I heard like, pow — it was like a bomb. It was like Hiroshima … amazing, amazing," said Carrier.

Fire officials said it was one of the most intense fires they've seen in years.