Australia declares Sydney church stabbing a 'terrorist' act

Australian police have said a brutal live-streamed knife attack at a church service in Sydney was a religiously motivated "terrorist" act as they urged calm from the angered local community.

"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 on Tuesday.

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".

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

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

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

Three other people were treated for non-stab wounds sustained in 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 re-entering 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 suspect's age as 15.

An AFP news agency journalist at the scene late on 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 have been deployed to the neighbourhood to protect local religious buildings.

Call for calm

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

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 are from 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."

There have also been schisms within the local Assyrian community over the teachings of the victim, Bishop Mar Mari Emmanuel.

Australians are still reeling from Saturday's stabbing, which was carried out by a 40-year-old man with a history of mental illness.

In that attack, videos shared on social media showed unshaven itinerant Joel Cauchi pursuing female victims as he rampaged through the vast, crowded Westfield shopping complex in Bondi Junction on Saturday afternoon. Cauchi also wounded a Pakistan-origin guard in the rampage.

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