Police say a large-scale anti-terrorism raid in Sydney this morning has foiled a plot to "commit violent acts" in Australia, including a plan to behead a member of the public.
More than 800 officers launched the raids as part of Operation Appleby in suburbs across Sydney's west and north-west, with a further 70 police involved in raids on properties in Brisbane's south.
Police said 15 people had been detained in Sydney as part of the operation between NSW officers, the Australian Federal Police and ASIO.
Court documents are expected to reveal that the raids, at 25 different properties, were aimed at a cell which planned to behead a member of the public in Sydney.
The documents are expected to say that the plan involved snatching a random member of the public in Sydney, draping them in an Islamic State group (IS) flag and beheading them on camera.
Prime Minister Tony Abbott said he was briefed last night on the operation, adding that the intelligence received by police gave "not just suspicion" but "intent".
"The exhortations, quite direct exhortations, were coming from an Australian who is apparently quite senior in ISIL to networks of support back in Australia to conduct demonstration killings here in this country," he said, using another acronym for IS.
"That's why the police and security agencies decided to act in the way they have."
Mr Abbott will cut short his visit to Arnhem Land today to farewell RAAF crews heading to the Middle East and to attend security briefings on the terror raids in Sydney.
'Right now is a time for calm'
He said the operation commenced earlier this year and had interrupted a terrorist attack in Australia.
"Police believe that this group that we have executed this operation on today had the intention and had started to carry out planning to commit violent acts here in Australia," he said.
"Those violent acts particularly related to random acts against members of the public."
Commissioner Colvin said the raids in Brisbane were not "directly linked" to the raids in Sydney, but authorities were looking to see whether there were any links.
He said the Brisbane raids were linked to a similar operation in Queensland last week, when an Islamic bookshop was searched, and two men arrested.
NSW Police Commissioner Andrew Scipione said the raids reflected "the reality of the threat we actually face".
"You know it is of serious concern that right at the heart of our communities we have people that are planning to conduct random attacks," he said.
"Today we work together to make sure that didn't happen. We have disrupted that particular attack.
"Our police will continue to work tirelessly to prevent any such attacks but certainly can I stress that right now is a time for calm.
"We don't need to whip this up."
He said cars were also searched in the raids and at least one weapon was seized.
Photo: AFP Acting Commissioner Andrew Colvin said the operation commenced earlier this year. (NSW Police)
A total of 25 search warrants were executed in the Sydney suburbs of Beecroft, Bellavista, Guildford, Merrylands, Northmead, Wentworthville, Marsfield, Westmead, Castle Hill, Revesby, Bass Hill and Regents Park.
Similar raids took place in Brisbane with officers conducting searches on properties at Creek Road in Mt Gravatt East, as well as Logan and Underwood.
The men have been accused of helping to recruit, facilitate and fund people to travel to Syria to engage in hostile activities.
Queensland Police Minister Jack Dempsey has sought to assure people in the state that they were safe.
"Obviously with the lead-in to G20 we're already at a certain risk level which ensures that Queenslanders are even safer than most other states and territories in Australia," he said.
Helicopters, loudspeakers involved in Guildford raid
ABC reporter Lucy Carter said part of Bursill Street in Guildford was still blocked off by police, where a home was being raided.
"[Police] are questioning a number of people on the front balcony of a single-storey home," she said.
"Neighbours say police burst into the house before dawn, shouting through loudspeakers and with a helicopter hovering overhead.
"Right now police are removing items including computers from this Guildford home, and a sniffer dog has also been brought in."
Neighbours said the occupants had only lived in the house for about three weeks.
A resident of Bursill Street, who wished to remain anonymous, has been trying to come to terms with the raids.
"I just find it so wrong for that to be happening here in Australia. How do we get to this stage that people are this out of control?" he said.
"You talk about the money going overseas - earlier this morning I heard $18 million or $20 million going overseas, and we don't know about it until it's too late."
Officers are refusing to give more information at this stage, as the operation is still underway.
They say they will provide updates throughout the morning.
Man claims he was punched by officer in raids
One man who was at one of the raided properties when police arrived claims to have been punched by an officer.
Photo: Maywand Osman said he was assaulted while being detained during the raids. (Supplied)
Maywand Osman, who was detained during the raids at Waterloo Rd in Marsfield but not arrested, said: "I opened the door this morning at 4:45am to about four police officers."
"They asked me to raise my hands. I immediately raised my hands. Four officers then jumped at me and one punched me in the face.
"They threw me to the ground and started hitting me in the head and pulling my hair.
"One officer grabbed me by the hair and said 'you piece of shit'. While they were beating me I heard one officer say 'just don't make him bleed'.
"They then went inside my house to conduct a search. They found nothing in my house and I was not under arrest or in custody at any point in time."
A statement released by Mr Osman's solicitor said: "My client was brutally attacked by four police officers this morning without provocation."
"He sustained injuries to the face and head. He was escorted to hospital by ambulance."
Commissioner Colvin said he was personally not aware of the claims.
Raids follow terror alert level raise: terrorism expert
Greg Barton, who heads the Global Terrorism Centre at Monash University, said the raids followed the warning issued by ASIO director David Irvine.
On Friday, authorities announced Australia's terror alert level had been lifted to high, meaning the risk of an attack is likely.
"He said there was considerable amounts of intelligence about plots underway and that's why he had to raise the terror alert level," Mr Barton said.
"Of course that begs the question: if they know about plots under the way what are they doing to intercept them? Now we're finding the answers today."
Video: Sydney counter-terrorism operation (YouTube: TheNSWPolice)
Immigration Minister Scott Morrison told the ABC's AM program that the raids demonstrate the "very real threat that's there".
"I think again [the operation] supports why the Government has been so strong in its response to this threat," he said.
He said the Government was working closely with the Islamic community "more broadly" and that "goodwill exists".
Communications Minister Malcolm Turnbull told Radio National this morning that the raids demonstrate Australian authorities are keeping the nation safe.
"Our security is the consequence of continued vigilance and hard work on the part of the security agencies," he said.
"There is no cause, no reason, for being complacent about security.
"There are people regrettably, some of them in our midst, that don't have the nation's best interests at heart."
Ikebal Patel from Muslims Australia told AM that the Islamic community has been stunned by the raids.
"Details are very sketchy and we don't even know who the individuals are and from which particular area, or sort of association they are part of," he said.
"So, it's all very very sketchy. It's all moving very fast."
The ABC understands the raids are linked to a similar operation in Queensland last week, when an Islamic bookshop was searched, and two men arrested.
The men have been accused of helping to recruit, facilitate and fund people to travel to Syria to engage in hostile activities.
Gallery: Anti-terrorism raids in Sydney, Brisbane
