Video of the attack shows the assailant, a tall man in jeans and a white baseball cap chase the priest around the altar and stab him.
Montreal: A priest was stabbed in front of his congregation live on television during morning mass on Friday at Canada's biggest church, police said.
Footage broadcast on a Catholic television station and picked up by the top-rated CTV network showed a man in jeans, a parka and a white baseball cap approach Claude Grou at St Joseph's Oratory in Montreal.
He is seen chasing the priest around the altar and standing menacingly over Grou as he falls to the ground, scattering candles and a banner, before stabbing his victim.
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."
"I'd thought he was just going to kneel in front of the alter to pray, like many people do," she said.
The suspect remained silent during the assault but appeared "very determined," Plamondon added.
Grou recovered enough to get to his feet and is seen backing away as security officers surround the assailant, who drops his knife. Three guards restrained him while others tended to Grou, who appeared to be in shock.
The priest was taken to hospital, where his condition was said to be stable, with police reporting to AFP that he had been "slightly wounded in the upper body."
The suspect was due to be interrogated shortly, while the church has been cordoned off, with services expected to resume later in the day.
Police did not reveal a motive for the attack.
Church spokeswoman Celine Barbeau said Grou was conscious when he was rushed from the scene.
"We are hopeful he will pull through." she added.
Mayor Valerie Plante called the assault a "horrible and inexcusable act that has no place in Montreal."
Saint Joseph's Oratory, built from 1920 to 1955, sits atop the north face of Mount Royal overlooking Canada's second-largest metropolis, and attracts some two million pilgrims and tourists each year.
The national shrine was founded by Brother Andre Bessette, a central religious figure among French-Canadians, who was canonized by Pope Benedict in 2010 for his devotion to the sick and disabled.
Jean-Francois Lefebvre, who was working in front of the oratory when the attack unfolded, said he saw police arrive and take away the suspect in handcuffs.
"I find this incident very unfortunate, in a place so peaceful that attracts millions of visitors," he told AFP.
"With what happened in Christchurch, it seems that all places of worship are being targeted," he added, referencing a gunman's attack at two mosques in New Zealand last Friday that left 50 dead and 50 wounded.
In 2017, a shooter gunned down six worshippers at a mosque in Quebec City.