WASHINGTON: It came out of nowhere, passengers on the Canadian Greyhound bus said. A young man was sleeping, head against the window, when the man sitting next to him began stabbing and then decapitating him as other travellers watched in horror.

The grisly and apparently random attack outside Portage La Prairie, Manitoba, on a bus bound for the capital, Winnipeg, ended with police apprehending the killer hours later, after a standoff.

The 37 other passengers, including several children, were left to relay their tale of the man wearing sunglasses who suddenly pulled out a hunting knife.

"We heard this blood-curdling scream and turned around, and the guy was standing up, stabbing this guy sitting next to him repeatedly, like 40 or 50 times," Garnet Caton, who was sitting in front of the victim, told journalists.

"When he was attacking him, he was calm - it was like he was at the beach. There was no rage … he was just like a robot stabbing the guy."

Terrified passengers fled from the bus and took shelter along the side of the road as the driver disabled the vehicle so the attacker could not escape.

When Mr Caton and the driver returned to the vehicle, they found the assailant still hacking the young man's body into pieces.

The severed head of the man, described as in his 20s, was displayed to the stunned passengers. "He went back to his seat and brought the head to the front and … displayed it to us … then dropped it on the ground." Mr Caton said.

Canadian mounted police took the passengers to a nearby hotel as others began an investigation. The identities of the victim and the suspect were not to be released until the families were notified.

A spokeswoman for Greyhound, Abby Wambaugh, offered condolences to the victim's family. "The incident was very tragic but was an isolated event," she said. "Bus travel has been and remains the safest mode of travel in the country."

The company said it would provide the passengers with any assistance, including trauma counselling and transportation.

Canadian officials had no explanation. The Minister of Public Safety, Stockwell Day, said the assailant should be prosecuted to the fullest extent of the law.

In my world this guy would have already ben killed no questions asked.... Death Penalty!!!