Australian security chiefs have declared Monday's attack on a bishop in Sydney an act of terrorism. 

Bishop Mar Mari Emmanuel, of the Assyrian Orthodox Church, was stabbed in his upper body and head during a livestream of a sermon at Christ the Good Shepherd Church in the Wakeley area of the city. 

The bishop has undergone surgery on his injuries. A priest, Father Isaac Royel, and two others were injured as they tried to stop the attacker. 

A 15-year-old boy has been arrested in connection with the attack. 

Tensions flared when large crowds gathered outside Christ the Good Shepherd Church to protest the attack, some clashing with police. 

This has prompted New South Wales premier Chris Minns to deploy thousands of police officers to religious organisations ''in order to avoid retaliation or tit-for-tat violence".

On Tuesday, the NSW government convened a meeting with faith leaders from across Sydney's religious communities who were invited to sign a statement calling for peace and calm.

Lyle Shelton, evangelical leader and National Director of the Family First Party, said, "The knife attack on Bishop Mar Mari Emmanuel in Western Sydney last night was shocking and hard to bear, particularly after the Bondi Junction massacre at the weekend.

"All such violence is truly evil. While anger and high emotions at the brazen attack on the Bishop is understandable, it was disappointing to see violence deployed by some. Violence is not the way and targeting and injuring police is reprehensible.

"Premier Chris Minns' calls for calm must be heeded."

Catholic Archbishop Anthony Fisher called the attack "shocking" and said it "has caused distress to many in the community".

That the attack took place on a religious leader during a service inside a church "has been especially confronting", he said, but he also condemned the subsequent violent protests.

"The images of the violence that subsequently occurred outside the church are also confronting," the Archbishop said. 

He added, "I urge the faithful to not respond to these events with fear, avoiding places of worship because they are worried about further attacks, nor with anger, engaging in acts of reprisal or revenge. The best response to violence and fear is prayer and peace."