A knife attack at a live-streamed church service in Australia wounded four people on Monday and sparked clashes between riot police and outraged members of the local community.

A bishop and the 15-year-old suspect were among those being treated for "non-life threatening injuries" after the attack at an Assyrian Christian church in the west of Sydney.

Police said the suspect is being held at an undisclosed location after his alleged attack provoked communal violence that also left two officers hospitalised.

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.

Graphic footage of Monday's stabbing showed the suspect emerging from the congregation, rushing the dais and slashing wildly at Bishop Mar Mari Emmanuel before being subdued by churchgoers.

Social media accounts leapt on news of the attack to make claims about the motive, none of which were verified by police.

There were tense scenes outside the church after Monday's attack, with hundreds of angry members of the local community trying to make their way past a phalanx of riot police to reach the suspect.

A journalist at the scene saw projectiles being hurled before police with riot shields and body armour pushed the protesters away from the church.

New South Wales acting assistant police commissioner Andrew Holland said the crowd had displayed an "emotional response to what took place in the church" – fuelled by rumours and graphic images circulating on the internet.

Two police vehicles and a number of houses were damaged as the protesters threw "bottles, bricks and other items," Holland said.

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."

Police urged the public to avoid the area amid "a large police response."

The video of Monday's attack was 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 swathe 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.

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 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. (AFP)