'I forgive whoever has done this': bishop after attack

In an audio message released by the church on Thursday, the bishop said he was doing fine and recovering quickly, while calling on followers to obey the law after a riot broke out in the aftermath of the attack.

"I need you to act Christ-like, the lord Jesus never taught us to fight," he said.

A 16-year-old has been arrested over the incident, but he is yet to be charged and remains under police guard in hospital.

"I forgive whoever has done this act, I say to him, you're my son, I love you, and I will always pray for you," Bishop Emmanuel said.

 "And whoever sent you to do this, I forgive them as well."

The Christ The Good Shepherd Church in western Sydney where a bishop was stabbed. (Bianca De Marchi/AAP PHOTOS)

Police have seized the boy's devices and are investigating whether he could have become radicalised in the lead-up to the attack, which senior officers say appears to have been religiously motivated.

Prominent Lebanese Muslim community leader Jamal Rifi said the teenager's mother did not believe he was radicalised online, adding that the 16-year-old regretted the incident.

"He kept on apologising to his mum and saying sorry and all that sort of thing," Dr Rifi told Sydney radio 2GB.

Bishop Emmanuel's call for calm comes after western Sydney religious leaders also tried to quell community tensions in the wake of the attack.

Police declared a terrorist act because the teenager allegedly made comments in Arabic that referred to insults against "my prophet" before stabbing the bishop, who had previously publicly criticised Islam and other religions.

More law-enforcement resources have been directed to western Sydney and around places of worship following the attack.

Police arrest a 19-year-old man over a riot outside a Sydney church where a bishop was stabbed. (HANDOUT/NSW POLICE)

One man has been charged over the riot outside the church, during which several police were injured and dozens of police vehicles were vandalised while a cohort of officers and paramedics were forced to shelter inside the place of worship.

The 19-year-old is scheduled to appear at Blacktown Local Court on Thursday charged with riot, affray and destroying or damaging property during a public disorder incident after he was arrested at a home in Doonside, 

Premier Chris Minns said more arrests were expected.

"There's been a major police task force that's been established to investigate this riotous behaviour ... that's a group of people that turned up to the church to throw objects and assault NSW police officers who were just doing their job," he told ABC TV.

More than 70 additional police officers have been deployed in Sydney's west to conduct high-visibility patrols.

Another 32 officers have been assigned to investigate the riot, during which some of the crowd tried to gain access to the alleged attacker while he was being held inside the church.

Police quelled a riot outside the church as people clashed with officers and each other. (Paul Braven/AAP PHOTOS)

Investigators are collating evidence including mobile phone and CCTV vision to identify those involved.

Police Minister Yasmin Catley said everyone had the right to go about their daily lives and practise their faith free from violence or intimidation.

"These extra boots on the ground will help provide calm and safety at a time when it's needed most," she said.