Screaming commuters were thrown from a double-decker bus when it ploughed into a passenger train at an Ottawa suburban crossing Wednesday, leaving six dead and scores injured.

Five people died at the scene -- including the bus driver, whose remains were so mangled they could not be immediately formally identified -- and a sixth in hospital.

Debris and bloody medical towels lay scattered trackside after emergency services mounted a large-scale rescue in a quiet district on the outskirts of the Canadian capital.

Thirty-four more were hurt and 10 people were still in a serious condition hours after the early morning commute turned to tragedy.

"We had bodies and debris pretty much everywhere at the impact site," fire department spokesman Marc Messier told broadcaster CTV earlier.

Ottawa Mayor Jim Watson later told a press conference the accident had been felt across Canada.

"We lost six of our neighbors, people who started off this bright, sunny day as we all did heading from their homes and loved ones to go about with their daily lives. And then this terrible tragedy struck," he said.

Ottawa police launched an investigation of the crash in the suburban neighborhood of Barrhaven, about 17 kilometers (10 miles) south of downtown Ottawa.

The Via Rail company operating the train on the Ottawa-to-Toronto line -- on which traffic has been suspended -- reported no fatalities. The train was on its way from Montreal.

The OC Transpo bus was headed downtown during the morning rush hour when the accident happened shortly before 8:48 am local time (1248 GMT).

At the time, the train was just 30 meters (100 feet) from a nearby station to pick up waiting passengers, on one of Canada's busiest rail corridors.

The front of the bus was mangled and shorn off while the locomotive and four rail cars were derailed by the crash that left parts of the track twisted.

Witnesses recounted panic aboard the bus just seconds before impact, which caused passengers to be thrown from the vehicle.

Several passengers braced themselves just prior to the collision.

Chad Mariage, on his way to work, was seated toward the back of the bus's second level when the accident happened.

He wasn't injured, he said, calling himself "one of the lucky ones."

"The impact was pretty severe," he told AFP. .

People on the bus were screaming just before the accident, he said, adding that the collision "wasn't a direct hit."

"We could all see the train coming towards us -- almost in slow motion," he said. "The bus driver hit the brakes but too late."

It was not immediately clear what caused the crash, which occurred on a cloudless and sunny autumn day in an area of wide open fields.

Lead investigator Jean Laporte of the Transportation Safety Board said his team was already documenting and photographing the scene of the accident.

They will assess the crossing -- which had been upgraded in 2005 after locals voiced safety concerns -- and its visibility lines, check signals and warnings and barriers to ensure they were working correctly, and analyze data from train recorder and the bus global positioning system, he said.

Prime Minister Stephen Harper expressed his condolences to the families of those killed.

"Deeply saddened to hear about the bus-train collision in Ottawa this morning," he tweeted. "Our thoughts and prayers are with the families of those involved."

Foreign Affairs Minister John Baird, a member of parliament for the affected district, praised the resiliency of the people of Ottawa in face of tragedy.

"I have no doubt that today, our city will mourn those we've lost, and support those in need as we move forward as a city, and as a community," he said in a statement.

Ottawa flags will be flown at half mast to honor the dead.

The accident happened just over two months after an oil tanker train derailed and exploded in the Quebec resort town of Lac-Megantic, killing 47 people.