A bus crash near Prescott, Ont. has left dozens of people injured, some critically, in hospitals across eastern Ontario.

The bus carrying 37 Chinese tourists travelling from Ottawa to Toronto smashed into a rock cut along Highway 401 around 2:45 p.m. according to Ontario Provincial Police.

Const. Suzanne Runciman said the passengers were injured by the force of the impact.

"The bus hit a rock cut area, therefore everybody on the passenger side of the bus would be in close proximity to that rock cut," she said.

OPP Const. Suzanne Runciman speaks to reporters at the crash site. ( Olivia Chandler/CBC)

Police said four people suffered life-threatening injuries and another 20 have suffered less serious injuries.

Runciman said there were no other vehicles involved.

"We don't know why the bus went off the road at this point."

The crash has closed Highway 401 westbound between Edward Street and Maitland Road. A school bus was brought in to bring the less seriously injured passengers to hospital. (Frédéric Pepin/Radio-Canada)

Paramedics were called to the scene from several neighbouring municipalities.

"We needed all hands on deck. There were a lot of casualties," said Runciman.

A spokesperson for the Brockville General Hospital said it had received five patients in critical condition and transferred a sixth patient to Kingston via Ornge air ambulance.

The Ottawa Hospital received one patient in critical condition and another two who were in serious but stable condition.

Dr. Bhaskar Gopalan, head of the Queensway Carleton Hospital's emergency department, said they initiated a "code orange," which allows them to call in more resources and staff.

"It lets everybody know there is a mass of patients coming into the department."

It was not immediately clear why the number provided by police is lower than the information from the hospitals.

Local resident Pud Johnston said the highway was quickly flooded with emergency workers.

"There were probably six ambulances here at the time, the helicopter was sitting on the 401, the traffic was still moving on 401," he said. "There were people lying on the shoulder of the road, there were groups of people watching and the medical people were doing their thing."

Resident Pud Johnston arrived at the scene the 401 bus crash shortly after it happened. 0:50

The Chinese Embassy in Ottawa said it had just become aware of the crash.

"We are following closely this accident and very much concerned about the conditions of the injured passengers. We are contacting the Canadian side for more information," the embassy said in an email.

The bus is operated by Union Tour Express, a company based out of Peabody, Mass.

Aiden Liang, a manager with the company, said the bus driver is in hospital and had a year of driving experience.

"I'm feeling very sad," said Liang. "There were many people injured on the bus."

Late Monday evening the OPP reopened the highway.