Three people are in critical condition and more than two dozen others injured after a bus filled with passengers rolled over on an Alberta highway Friday.

Passengers described a horrifying, bloody scene after the bus tumbled and skidded to a halt, tossing people around.

It took emergency crews half an hour to free five victims trapped in the wreckage after a Red Arrow charter bus rolled over on Highway 28 near Redwater, Alta., around 2:15 p.m. local time.

A dozen ambulances from seven nearby communities, along with firefighters and police, rushed to the scene. A witness told CTV Edmonton some people had escaped through an emergency hatch on the roof of the bus while others smashed the windshield to get out.

Lalo Ramkissoon told The Canadian Press it felt like the bus was speeding and wobbly as it took a curve before it tipped. He braced his wife with one arm and blocked falling bags from hitting her with the other.

Ramkissoon said windows shattered as the bus skidded along the pavement, and he saw some people's faces scrape along the road.

"There was just blood everywhere," and people were screaming, passenger Daniel Chingombe told CP.

After he managed to climb out a window, Chingombe said he saw a severed hand on the ground.

Three people were taken to an Edmonton hospital in critical condition, while more than 20 others were taken to various hospitals in the area with less serious injuries, reported CTV Edmonton's Bill Fortier. Several others were treated at the scene.

Two of the seriously injured victims are in their 60s, a STARS air ambulance spokesman told The Canadian Press.

There were some emotional reunions on the highway as those who were able to walk away from the crash embraced loved ones who came to pick them up.

RCMP Sgt. Patrick Webb told CTV News Channel most people were able to escape with minor injuries.

Webb said it's not yet clear why the bus rolled over. Police will interview the driver and passengers as soon as possible, he said. The bus driver seemed alert and was talking to paramedics, Fortier reported.

Police are looking into all possible causes, including a slippery curve in the highway and driver fatigue.

Red Arrow general manager John Stepovy said all 36 seats on the overflow bus travelling the regularly scheduled Fort McMurray-Edmonton route were taken.

"Our main concern in the well-being of passengers on the coach," he said.

Highway 28 was closed for several hours and drivers were asked to avoid the area.