A priest was stabbed during a livestreamed morning mass Friday at Canada's biggest church, Saint Joseph's Oratory in Montreal, police said.

The suspect was arrested at the Roman Catholic basilica shortly after the attack in front of 50 people and a television audience around 8.40am (1240 GMT), while priest Claude Grou was taken to hospital.

Police told AFP that Grou was "slightly wounded in the upper body" when, according to witnesses cited by authorities, "the suspect suddenly ran up to the priest and attacked him with a knife" while he was officiating the mass.

The suspect was due to be interrogated shortly, while the scene has been cordoned off.

Church spokeswoman Celine Barbeau said Grou, the church's rector, "was conscious when he left. We are hopeful he will pull through".

Video of the attack rebroadcast by CTV showed the assailant - a tall man in jeans, a parka and a white baseball cap - chase the priest around the altar and stab him.

Grou falls to the ground, but quickly gets back up and backs away as security officers surround the suspect and detain him until police arrive.

A witness, Adele Plamondon, told public broadcaster Radio-Canada the priest was about to start reading the gospel, when "this man drew a knife and ran up to stab him."

The suspect "did not shout, didn't say anything" during the assault, but appeared "very determined," Plamondon said.