A stabbing attack at a Sydney church on Monday night has been declared a terrorist incident by NSW Police.

A bishop was stabbed at the altar during a live-streamed church service in Sydney, in another horrendous incident that triggered “uproar” among a mob that gathered outside.

Police arrested a 15-year-old boy after he allegedly attacked Bishop Mar Mari Emmanuel at the Orthodox Assyrian church in Wakeley.

The religious leader was allegedly stabbed multiple times in the head and upper body at Christ the Good Shepherd Church in front of the shocked congregation about 7.10pm.

In footage streamed live on the church’s social channels, a boy in a black hoodie can be seen approaching the bishop with a knife in his hand and repeatedly thrusting his arm in a stabbing motion.

Police said three worshippers were also injured as they ran to Emmanuel’s aid. None suffered life-threatening injuries and were taken to hospital.

The bishop remains in a critical but stable condition. He was in surgery on Tuesday morning.

Also onTuesday, NSW Police Commissioner Karen Webb said the incident was being investigated as a terrorist attack.

It was the second stabbing in Sydney in as many days after Joel Cauchi murdered six people at Westfield Bondi Junction on Saturday afternoon.

The latest attack sparked unrest in the local area. Hundreds of angry people gathered outside the religious venue in the aftermath, clashing with police who responded to the incident.

Police respond to the unrest in Wakeley on Monday night. Photo: AAP

NSW acting Assistant Commissioner Andrew Holland said the violent attack, which was broadcast live on the church’s social channels, was “probably what caused the uproar in the community”.

“People saw that, responded and unfortunately we ended up with a public order incident,” he said.

Holland said a 15-year-old suspect “sustained injuries to his hand as a result of his actions”.

He said the stabbing suspect was held down by parishioners of the church until police arrived.

“Police were in the process of conveying the young person from the church when it was identified a large group had gathered outside,” Mr Holland said.

“A decision was made to retain the young person in the church for his safety”.

About 100 NSW Police officers outside the church after Mondays stabbing. Photo: AAP

The crowd continued to build outside the church and “a public order incident developed requiring police from all regions across Sydney”.

Holland said more than 100 police officers and 30 police vehicles attended. The arrested teenager was later removed from the church.

Two police officers were taken to hospital after being injured by members of the crowd, who police say broke into “houses to gain weapons to throw”.

Several police cars were damaged.

Holland said some in the group also threw items at the church to try to get to the teenager inside.

“They need to let police to do their jobs and let us do the investigation,” he said.

“Police will make arrests in the future in relation to this public order incident.”

NSW Premier Chris Minns urged calm, adding that he convened a meeting of faith leaders representing religious communities across western Sydney late on Monday.

He said they “endorsed and supported a unanimous condemnation of violence in any form, called for the community to follow first responder and police instructions and called for calm in the community”.

“We’re calling on everyone to act with kindness and respect for each other,” he said.

NSW Ambulance said it was called to the church about 7.15pm. It sent multiple paramedics.

NSW Police said two clergyman were taken to hospital.

The church said Emmanuel and a senior priest were in a stable condition and also appealed for calm.

“We ask for your prayers at this time,” the church said in a statement on social media.

“We also kindly ask anyone at the church to leave in peace as our Lord and Saviour, Jesus Christ, teaches us.”

-with AAP