Anthony Albanese has pleaded for unity after a terrorist attack at a western Sydney church where a bishop and a priest were stabbed, and a subsequent riot in which police officers were attacked.

The prime minister held a joint press conference with police and security chiefs in Canberra on Tuesday morning, hours after New South Wales declared as a terrorist attack the stabbing of Bishop Mar Mari Emmanuel at a service at the Assyrian Christ the Good Shepherd church in Wakeley just after 7pm on Monday.

Counter-terrorism investigators – a joint team comprising NSW and federal police as well as the Australian Security Intelligence Organisation (Asio) – now have extraordinary powers under NSW laws to investigate the attack, as well as conduct searches to prevent any further suspected attacks.

A live stream of the service on the church’s website showed a person approaching the altar who then appeared to stab toward the bishop’s head multiple times.

The congregation then swarmed forward, with a scuffle ensuing between the worshippers and the attacker. Police arrested a 16-year-old and were forced to hold him at the church for his own safety as a large crowd of several hundred people gathered outside the church. Riot police were called in to forcibly move the crowd on after police cars were smashed.

The NSW premier Chris Minns gathered leaders of the local Muslim, Assyrian and Melkite communities for an emergency meeting at 10.30pm on Monday, organising for them to put their names to a statement condemning the violence and calling for calm.

However tensions remained fresh on the ground as Minns, local MPs and the Fairfield mayor visited near the site of the incident.

Meanwhile, leaders of Lakemba mosque in Sydney’s west revealed they had received threats to firebomb the mosque on Monday night, and would have heightened security over the next week.

Albanese on Tuesday expressed his sympathies to the Assyrian community in western Sydney. “This is a disturbing incident. There is no place for violence in our community.”

“There’s no place for violent extremism. We’re a peace-loving nation. This is a time to unite, not divide, as a community, and as a country.”

“We have overwhelmingly a harmonious society in Australia. In my local community, people of different faith live side by side. And that is overwhelmingly the experience of Australians. It is vital, in my view, that we continue to stress what unites us, and that respect for each other be maintained at all times.”

Albanese declined to state the religion of the attacker, but Asio chief, Mike Burgess, at the joint press conference with the prime minister, said he was aware of video of the alleged offender speaking in Arabic.

“If he [the bishop] didn’t get himself involved in my religion, if he hadn’t spoken about my Prophet, I wouldn’t have come here … if he just spoke about his own religion, I wouldn’t have come,” the alleged attacker can be heard saying in the video.

The video reportedly shows the mayhem that followed the attack, with people in bloodied clothes walking around as the alleged attacker is held against the ground.

Emmanuel, who has a popular online presence, has previously criticised Islam and the prophet Muhammad in public sermons.

On the declaration of the event as a terrorist attack, Burgess said: “To call it a terrorist act, you need indications of information or evidence that suggest actually the motivation was religiously motivated or ideologically motivated.

“In the case of Saturday [the stabbing at Bondi Junction], that was not the case. In this case, the information we have and the police have before us indicates that is strongly the case. That is why it was called an act of terrorism,” Burgess said.

By declaring the event a terrorist attack, police will have greater investigative powers under NSW’s terror laws. It includes powers to search properties and vehicles, among other methods, to examine a past attack or prevent a suspected one from occurring.

He said that despite the declaration of a terrorist attack, the current terror level threat for Australia – “possible” – would not be raised. “One incident like this does not change the threat level but we keep it under review.”

Burgess said that while there were no indications of others connected to the attacker, Asio was investigating to determine there were no further threats.

The Australian federal police commissioner, Reece Kershaw, also at the joint press conference, called the subsequent commotion “a disgraceful act from the community who attacked police at that scene”.

“My support goes to the NSW commissioner and the NSW police officers who were there to protect the community,” Kershaw said. “It was really un-Australian to see that happen last night.”

Minns said the decision to make the terror declaration was taken early on Tuesday morning and validated by the police minister.

The NSW police commissioner, Karen Webb, said a strike force had been established to investigate the incident.

“We’ll allege there’s a degree of premeditation on the basis this person has travelled to that location, which is not near his residential address, he has travelled with a knife and subsequently the bishop and the priest have been stabbed,” Webb said.

Webb said the stabbing victims “are lucky to be alive”.

Webb said that after the stabbing a crowd of people then “converged on that area and began to turn on police”. Police estimate the crowd grew from 50 people to approximately 500.

A finger of the alleged attacker was severed during the incident. Webb said it was “not clear yet whether it’s self-inflicted or through the melee that occurred or some other way, and I understand that it’s possibly a flick knife, but that is to be confirmed as well”. He later underwent surgery.

“People used what was available to them in the area, including bricks, concrete, palings, to assault police, and throw missiles at police and police equipment and police vehicles.”

Some police officers were injured and taken to hospital overnight, while 20 police vehicles were damaged and 10 rendered unusable, Webb said.

“That is unacceptable and those that were involved in that riot can expect a knock at the door. It might not be today. It might not be tomorrow. But we’ll find you and we’ll come and arrest you. That is totally unacceptable.”

The NSW ambulance commissioner, Dominic Morgan, said 30 patients had been assessed and treated overnight, with seven taken to hospital, about 20 of them having been affected by capsicum spray.

The bishop and priest who were stabbed were in surgery on Tuesday morning.

Paramedics had come “directly under threat” and had to retreat into the church during the riot, with six of them stuck in the church for three and a half hours, Morgan said.

The decision to designate the stabbing incident a terrorist attack was made in the early hours of Tuesday morning, Minns said, validated by the police minister at 1.45am and the premier himself notified at 2am.

The alleged offender had not previously been on any terror watch list.

Albanese warned it was “completely not acceptable” to damage or impede police vehicles, as occurred after the incident.

A separate strike force, called Dribs, had been established to investigate the riot, and additional police would be patrolling Sydney in response.

The incident occurred just days after another man, Joel Cauchi, killed six people and injured others, some critically, in a stabbing spree at a Westfield shopping centre in the Sydney suburb of Bondi Junction. Investigators do not believe Cauchi’s attack was terrorism related.

Minns called for community calm on Tuesday.

“I understand, coming just days after the criminal activities in Bondi Junction, Sydney and NSW is on edge and there’s understandable community anxiety at the moment. However, it’s crucially important the police are able to conduct their inquiries and finish their investigation,” Minns said.

Minns said any further violence would be diverting resources away from the investigation of the incident.

“I want to make something very, very clear – there’s no such thing in Australia as taking the law into your own hands. It doesn’t exist. That’s for several reasons. Firstly, you will be met by the full force of the law if there’s any attempt for tit-for-tat violence in Sydney over the coming days.

“Lastly, and perhaps most importantly, every religious leader representing communities across western Sydney has expressly said don’t do it, and implored the community to come together to express faith and hope in the NSW police, and ensure that we remain united during these difficult days.”