Montreal police have released a security video of three young men suspected of assaulting a bus driver early Wednesday morning, putting him in hospital in serious condition with head injuries.

Police said the assault happened on the 363 night bus as it travelled along St-Laurent Boulevard.

The video, from a security camera mounted at the front of the bus, was taken as three men, all aged around 20, got on the bus at the corner of St-Laurent Boulevard and Fairmount Avenue.

Police said the passengers were annoyed the bus stopped a short distance away from where they were waiting, and they were verbally abusive to the driver. They went to the back of the bus, but at about 2:30 a.m., as the vehicle neared Jean-Talon Street, they got up to leave by the front door, insulting the driver once again and spitting on him.

That's when police say the dispute escalated, and the driver was attacked.

Police are looking for a white man about 5-foot-10, a black man, 5-foot-8, and a third man, 5-foot-8, heavy-set, also black.

Anyone with information that could help identify the suspects is asked to contact police at (514) 280-2444.

'Senseless' attack: drivers' union

A spokesman for the union representing the bus driver said the Société de transport de Montréal (STM) called the attack senseless.

Tom Mouhteros said the STM has taken a number of measures to protect drivers in recent years, but he added such assaults are difficult to prevent.

"They come out of nowhere," said Mouhteros. "Can you stop an attack? I don't think anybody can stop an attack."

STM spokeswoman Isabelle Tremblay said increased security measures including more training for drivers have helped to repel assaults.

"We put a lot of energy into training our drivers to interact with people and try to undo certain potential difficulties," Tremblay said.

She said the number of verbal and physical assaults on drivers dropped by 50 per cent between 2007 and 2012.

Buses are now equipped with panic buttons for drivers, and many are equipped with cameras. As well, STM security patrols occasionally follow overnight buses along their routes.