A 15-year-old male has been arrested after at least four people, including a prominent and controversial bishop, were injured in a stabbing at a church in Sydney on Monday, according to Deutsche Welle.

Police said they were treating the attack as a terror incident.

"We believe there are elements that are satisfied in terms of religious motivated extremism," New South Wales state Police Commissioner Karen Webb said at a press conference on Tuesday.

"After consideration of all the material, I declared that it was a terrorist incident."

A 50-year-old man was taken to hospital in a serious condition while three others were treated at the scene for "non life-threatening injuries," according to the New South Wales state ambulance service.

The stabbing was carried out during a service at the Good Shepherd Church in the suburb of Wakely, 30 km (18 miles) west of Sydney's central business district.

The congregation belongs to the Assyrian Church of the East traditionally based in parts of the Middle East and southern India.

A video on social media shows a man approaching Bishop Mar Mari Emmanuel at the altar of the church and appearing to stab him repeatedly.

A separate eyewitness video, taken in the aftermath of the incident and verified by the Reuters news agency, shows the man being pinned to the ground by several others, his face obscured.

A voice can reportedly be heard saying in Arabic: "If they didn't insult my prophet, I wouldn't have come here. If he didn't involve himself in my religion, I would not have come here."

Police say the suspect is cooperating, but that he had to be held inside the church by officers to protect him from vengeful members of the congregation.

"There was so much anger because the bishop is loved by them, he’s loved by myself as well," a local resident who gave her name as Canny told Reuters.

"He preaches about the Lord and we love the Lord," she said.

Outside the church, angry worshippers reportedly threw stones at police, injuring two officers, who responded with tear gas.

"A large police response is underway and the public is urged to avoid the area," police said.

Chris Minns, the Premier of the state of New South Wales of which Sydney is the capital, described the scenes as "disturbing" and urged the community to "stick together in the face of adversity."