SYDNEY - The stabbing attack at a church in Sydney overnight was a religiously motivated "terrorist" act, police said on Tuesday.

New South Wales (NSW) Police Commissioner Karen Webb said she determined the stabbing was a terrorist incident after "consideration of all the material".

The stabbing attack, in which two church leaders were wounded, was deemed an act of religiously motivated "extremism" that intimidated the public, Webb told a news conference.

A spokesperson from the NSW Ambulance confirmed that four people were injured in the attack.

A teenager remained in police custody following the alleged stabbing at the church in Sydney's southwest overnight, NSW Police Force said in a statement.

Police were called to the church on Welcome Street, Wakeley, following reports of a stabbing. A 15-year-old boy, who had been restrained by members of the public, was arrested by police.

A large crowd gathered outside, with further police from across the Metropolitan Region, including specialist units attending.

During a public order incident, projectiles were thrown at officers, during which a number of police officers were injured, police vehicles damaged, and property stolen, according to the police statement.