Police are treating an apartment building fire in St. John's Wednesday night as suspicious.

No one was injured and only one basement unit was damaged in the fire that broke out shortly before 10:30 at Freshwater Road Apartments.

Firefighters put it out quickly, but smoke filtered through the halls of the three-storey, 16-unit building, forcing the evacuation of dozens of people out into the rain.

David Joy, who lives on the top floor, said he knew something was wrong when he first heard the alarm.

"It got louder and louder ... I woke up my sister and I said, 'Julie we got to get out,' " said Joy, who grabbed his cat and fled the building after someone started pounding on his door with an evacuation call.

Another resident, David Reynolds, said there was a lot of smoke.

"When we exited the building through the basement, just a few windows from the exit, we could see smoke billowing out," he said. "There was a lot of black smoke. It smelled a lot like burning plastic."

A woman and her nine-year-old son were inside the apartment where the fire started, but Andrew Murray and Justin Ryall, two residents from a neighbouring unit, came to her rescue.

"We were both ready to go to bed, and we heard the alarm in one of the rooms down the hallway. And I didn't think much of it, but he went and had a look," Murray said of Ryall.

Ryall started knocking on the door.

"And she said, 'Help me,' " he said.

Ryall summoned Murray to help.

"[Justin] was giving the door a few boots, and it wasn't going down. So he said, 'I'll go around to the window and you try and get her out through the door,' " Murray said.

"I did get through the window," said Ryall, "but when I got in there, they were already gone."

"They're OK," said Derek Chafe, a superintendent with the St. John's Regional Fire Department. "[But] there's a lot of damage done to the apartment, a lot of fire damage."

The Canadian Red Cross is helping the woman and her son with emergency housing and food.

The other tenants were outside for about an hour while firefighters put out the blaze and smoke was cleared from the building.

The Royal Newfoundland Constabulary continue to investigate cause of the fire.