Australian police on Tuesday said a brutal knife attack during a live-streamed church service was a religiously motivated "terrorist" act, as they urged calm from the angered local community.

Two people were stabbed when a 16-year-old suspect rushed the dais at an Assyrian Christian church in western Sydney late Monday, slashing wildly at the bishop who was giving a sermon.

The bishop was stabbed in the head and chest and taken to hospital.

The attacker was immediately subdued by outraged congregants and later taken into police custody.

He was "known to police" but was not on any terror watchlists, senior officers said.

"After consideration of all the material, I declared that it was a terrorist incident," New South Wales police commissioner Karen Webb told a news conference.

Webb said the attack was deemed an act of religiously motivated "extremism" that intimidated the public -- adding that the victims were "lucky to be alive".

The head of Australia's leading spy agency said that the suspect appeared to have acted alone and there was no immediate need to raise the country's terror threat level.

"At this stage, it looks like the actions of an individual," Australian Security Intelligence Organisation head Mike Burgess said in rare public comments.

Three other people were treated for non-stab wounds sustained as a result of the attack and about 30 more were treated after a riot that ensued outside the church.

For three hours, more than 500 protestors clashed with a phalanx of riot police who battled to prevent them from reentering the church and lynching the teen.

He is now being held at an undisclosed location and is believed to have also sustained knife injuries. Authorities had originally given the his age as 15.

An AFP journalist at the scene late Monday saw projectiles being hurled before police with riot shields and body armour eventually pushed the protesters away from the church.

Twenty police vehicles and some houses were damaged as the protesters threw bottles, bricks and other items.

One officer was "hit with a metal object and sustained a twisted knee and a chipped tooth", police said.

"Another constable sustained a broken jaw after he was hit with a brick and a fence paling."

Calm was eventually restored, but more officers were deployed to the neighbourhood to protect local religious buildings.

The church said in a statement Tuesday that the victim, Bishop Mar Mari Emmanuel, was in a stable condition and "improving".

- Call for calm -


AFP verified the video of Monday's attack as being taken at the Christ the Good Shepherd Church, which has an online following of almost 200,000.

It is located in Sydney's western suburb of Wakeley, a hub for Sydney's small Christian Assyrian community, many of whom fled persecution and war in Iraq and Syria.

New South Wales state premier Chris Minns issued a joint statement with Christian and Muslim leaders calling for calm.

"We're calling on everyone to act with kindness and respect for each other", the statement said.

"Now is the time to show that we are strong and united."

Assyrians trace their roots to an empire that ruled a swath of the ancient Middle East.

Since that age of empire, they have faced persecution as a religious and ethnic minority distinct from the region's Arab and Muslim majorities.

There have also been schisms within the local Assyrian community over the teachings of the bishop who was attacked.

The incident came two days after a 40-year-old man with a knife killed six people at a shopping mall in the city's east before being shot dead by police.

In that attack, videos shared on social media showed unshaven itinerant Joel Cauchi, who had a history of mental health, pursuing mostly female victims as he rampaged through the vast, crowded Westfield shopping complex in Bondi Junction on Saturday afternoon.

A black ribbon was projected onto the Sydney Opera House on Monday as a mark of respect for the victims of that attack.