A knife attack at a western Sydney church that left two clergymen in hospital is being treated as a terrorist act.

A 15-year-old boy is in custody on Tuesday after the overnight attack at Christ the Good Shepherd Church in Wakeley, where Bishop Mar Mari Emmanuel was stabbed during a live-streamed sermon.

A church priest was also injured after trying to intervene.

After a series of early morning phone calls involving the NSW Police commissioner, police minister and premier, the incident was declared a terrorist act.

Commissioner Karen Webb said the teenager made comments that pointed to a religious motive as he launched the attack.

“After consideration of all the material, I declared that it was a terrorist incident,” she told reporters on Tuesday.

Footage of the incident showed a person dressed in black approaching and stabbing the bishop multiple times to the head and upper body at the altar as parishioners screamed and ran to the cleric’s aid.

“We’ll allege there’s a degree of premeditation on the basis this person has travelled to that location, which is not near his residential address, he has travelled with a knife and subsequently the bishop and the priest have been stabbed,” Ms Webb said.

“They’re lucky to be alive.”

The teenager was known to police but was not on a terror watch list, the commissioner added.

The weapon used has been described as a flick knife and detectives are also investigating if the boy lost fingers as part of the attack or the unrest afterward.

Prime Minister Anthony Albanese was also briefed on the incident before calling a meeting of the National Security Committee, to be held later in the day.

The Australian Federal Police and ASIO earlier briefed Mr Albanese on the attack.

The attack led to street violence near the Assyrian church, where hundreds of people gathered and scores of police tried to disperse the crowd during which officers were injured and vehicles damaged.

About 30 people were treated over the course of several hours at the location, including seven who were taken to hospital, NSW Ambulance said.

Multiple police officers were injured at the scene.

Acting Assistant Commissioner Andrew Holland told reporters on Monday night the violent images captured inside the church probably caused the uproar in the community.

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

Mr Albanese said it was important not to speculate on the motivations for the attack during the early stages of the police investigation.

“What I know is that there’s no place for violence in our community,” he told ABC radio.

“There’s no place for violent extremism. We’re a peace-loving nation.”

The church said Bishop 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 posted on social media.

Mr Holland said the teenaged 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,” he said.

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

More than 100 police officers and 30 police vehicles attended and the suspect was later removed from the church.

Members of the crowd broke into several houses for weapons that were then thrown at police, Mr Holland said.

Items were also thrown at the church in a bid to get access to the suspect.

Police expect to make arrests over the stabbing aftermath as part of an investigation that will run in parallel to their probe into the church attack.

The attack came two days after a mass stabbing at the Westfield Bondi Junction shopping centre, in Sydney’s east, that left six people dead and more than a dozen injured.

Police have said that attack, carried out by a mentally ill Queensland man, was not thought to be terror-related.

Source: AAP