SHERBROOKE, Que.--Two people were killed as an explosion loud enough to be heard for kilometres ripped through an industrial plant Thursday, triggering a fire that produced a thick, dark cloud of toxic smoke.

The blast and fire sent 19 other people to hospital, some with severe burns. The two victims were found in the rubble of the decimated processing plant.

The incident occurred in Quebec's Eastern Townships at the Sherbrooke facility belonging to Neptune Technologies & Bioressources, which produces health products such as Omega-3 derived from marine life.

The local 911 line was flooded with a record number of calls, authorities said.

Martin Carrier, a Sherbrooke police spokesman, said more than 100 people in Sherbrooke and surrounding suburbs phoned within a minute.

"They heard the explosion," Carrier said. "It was a big noise. A lot of black smoke. You could see it everywhere in the city."

When first responders arrived at the plant, they beheld a scene of devastation as workers fled for safety.

"We've got people injured inside, we've got people injured outside," Carrier said.

"Some were walking, helped by another. It was a chaotic scene," he said. "Pretty tough."

Firefighters probed the tangled building carefully, looking for potential victims. Among the 19 injured, four were transported to a burn unit in Montreal; two were in an intensive-care unit in Sherbrooke; seven were quickly released from hospital; and six were held for observation. The bodies were found later in the day.

It was the first of two major fires Thursday in Quebec.

Hours later, flames were skipping off a rooftop in Old Montreal as emergency crews were called in to deal with a blaze a block away from the famous Notre-Dame Basilica, close to the Montreal La Presse newspaper which had to be evacuated. There were no reports of injuries in the Montreal fire.

But the Sherbrooke blaze was potent enough that smoke kept drifting up for hours, even after fire crews had contained the flames, while an acrid stench continued to hover in the air.

The plant lay in ruins. The only walls that remained standing were scorched black by the flames.

"Just looking at the damage to the building you can see it was probably very violent," said Gaetan Drouin, head of the local fire service.

"Even before the 911 calls came in (one fire station) already had many firefighters on the way. They set off the alarm bells just from seeing the plume of smoke that shot up into the sky."

Fears about toxic smoke stemmed from the plant's 15,000-litre reserve of acetone, a flammable substance consumed by the fire.

When ingested, acetone can cause irritation. However, local health officials downplayed the toxic threat, suggesting people might potentially experience headaches or nausea but little else because of the smoke.

Environment Quebec said it was actually more concerned about the possibility of soil or water contamination than of the risk associated with breathing the air near the blast site.

It's unclear what caused the explosion.

There was at least one large blast followed by a series of smaller ones. Police set up a security perimeter and cleared the area around the industrial park.

One man who lives nearby said: "I was working in the garage and I heard a loud, 'Boom.' Then we were evacuated." Another neighbour said the flames were visible from blocks away. Both declined to give their names.

Within minutes of the blast, police said, employees had been evacuated from the facility and supervisors were performing a head count outdoors.

Residents were asked to avoid the area around the industrial park. "We're asking people to stay away... These are possibly toxic clouds," said Rene Dubreuil of the Sherbrooke police.

The Laval-based company announced plans last year to expand its Sherbrooke facility. The federal government supported the project with an interest-free loan and the Quebec government provided a grant, according to an announcement made earlier this year by then-premier Jean Charest.

Shares of the company plunged 10 per cent during a sell-off in less than a half-hour of trading following the incident.

The company called in a psychiatric support team to help traumatized workers. It promised to co-operate with police during the investigation.

"We're in terrible shock over what's happened," said Michel Chartrand, chief of operations for the company.