Australian authorities have six people in custody and say they have identified a suspected ringleader after thwarting an ISIS-inspired plot to carry out random beheadings in Sydney and Brisbane.

Raids were conducted on more than a dozen properties across Sydney involving 800 federal and state officers. One of the detainees has been charged with conspiracy.

The ringleader wasn't named, but Mohammad Ali Baryalei, who is believed to be Australia's most senior ISIS member, was named as a co-conspirator in court documents filed later Thursday. Baryalei, 33, is a former Sydney nightclub bouncer and part-time actor.

Australian Prime Minister Tony Abbott: police raids came in response to intelligence that ISIS in the Middle East was calling on Australian supporters to behead a random person in Sydney. (Associated Press)

He is understood to have made the instruction to kidnap people in Brisbane and Sydney and have them executed on camera. That video recording was then to be sent back to ISIS's media unit, where it would be publicly released, the Australian Broadcasting Corp. reported.

Prime Minister Tony Abbott confirmed that the detainees, followers of the Islamic State in Iraq and Syria, were planning to seize a random person in Sydney and behead him or her.

"That's the intelligence we received," he told reporters. "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 used the name ISIL to describe the al-Qaeda splinter group known as the Islamic State of Iraq and the Levant, or simply as Islamic State.

The raids came in response to intelligence that an Islamic State group leader in the Middle East was calling on Australian supporters to behead someone.

"This is not just suspicion, this is intent and that's why the police and security agencies decided to act in the way they have," Abbott said.

A member of the Australian Federal Police forensic unit inspects a garage at a house that was involved in pre-dawn raids in the western Sydney suburb of Guilford on Thursday. (David Gray/Reuters)

Attorney General George Brandis confirmed that a person born in Afghanistan who had spent time in Australia and is now working with the Islamic State group in the Middle East ordered supporters in Australia to behead people and videotape the killings.

"If the … police had not acted today, there is a likelihood that this would have happened," Brandis told the Australian Broadcasting Corp.

New South Wales police did not say why nine of the detained people were released, or whether they would face charges later.

The raids came just days after the country raised its terrorism threat to the second-highest level in response to the domestic threat posed by supporters of the Islamic State group.

At the time, Abbott stressed that there was no information suggesting a terror attack was imminent.

One of those detained, 22-year-old Omarjan Azari of Sydney, appeared briefly in a Sydney court on Thursday.

Prosecutor Michael Allnutt said Azari was involved in a plan to "gruesomely" kill a randomly selected person — something that was "clearly designed to shock and horrify" the public. That plan involved an "unusual level of fanaticism," he said.

Azari is charged with conspiracy to prepare for a terrorist attack. The potential penalty was not immediately clear.

In court documents, Azari is accused of conspiring with Baryalei and others between May and September to prepare for a terrorist attack. Allnutt said the charge stemmed from the interception of a phone call a couple of days ago.

Azari did not apply for bail and did not enter a plea. His next court appearance was set for Nov. 13.

His attorney, Steve Boland, said during the hearing that the allegation against his client was based "on one phone call." He did not speak to reporters outside court.

Invading privacy

Dozens of police spent Thursday searching Azari's home and a car parked across the street from his house. One officer pulled a memo out of the car from the Australian National Imams Council outlining concerns about Australia's new anti-terrorism proposals.

The council did not immediately respond to messages seeking comment. On its website, it says proposals to expand government surveillance powers would invade privacy. It also denounces the Islamic State group, saying there "is nothing Islamic about their murderous actions."

A second man was charged Thursday night in connection with the raids. The 24-year-old, whom police didn't name, was charged with possessing ammunition without a licence and unauthorized possession of a prohibited weapon. He was released on bail and ordered to appear in court next week.

The Australian Security Intelligence Organization's director-general, David Irvine, said the threat of terrorism in the country has been rising over the past year, mainly due to Australians joining the Islamic State movement to fight in Syria and Iraq.

'Time for calm'

Some terrorism experts question, however, whether the extremist group has the capacity to organize a major terror campaign in Australia, far from its base.

Police declined to reveal exact details of the attack they believe was being plotted. New South Wales Police Commissioner Andrew Scipione said only that it was to be carried out against a member of the public on the street and was at "a very high level."

"Right now is a time for calm," Scipione said. "We need to let people know that they are safe, and certainly from our perspective, we know that the work this morning will ensure that all of those plans that may have been on foot have been thwarted."

A separate series of raids was conducted Thursday in the eastern cities of Brisbane and Logan. Last week, Australian police arrested two men in Brisbane for allegedly preparing to fight in Syria, recruiting jihadists and raising money for the al-Qaeda offshoot group Jabhat al-Nusra, also known as the Nusra Front.

Federal Police Deputy Commissioner Andrew Colvin said the raids conducted in Brisbane on Thursday were a follow-up to that operation. Queensland Police Commissioner Ian Stewart said the operations in Sydney and Brisbane were linked, but declined to release details.

60 fighting in Iraq, Syria

Police said at the time there was no terrorist threat to the Group of 20 leaders' summit to be hosted by Brisbane in November that will bring President Barack Obama and other leaders of the world's 20 biggest economies to the Queensland state capital.

Australia has estimated that about 60 of its citizens are fighting for the Islamic State group and the Nusra Front in Iraq and Syria. Another 15 Australian fighters have been killed, including two young suicide bombers.

The government has said it believes about 100 Australians are actively supporting extremist groups from within Australia, recruiting fighters and grooming suicide bomber candidates as well as providing funds and equipment.

A Sydney money transfer business owned by the sister and brother-in-law of convicted terrorist Khaled Sharrouf, an Islamic State fighter, had its license suspended this week on suspicion it had been sending 1 million Australian dollars ($900,000 US) a month to the Middle East to finance terrorism, said John Schmidt, chief executive of the industry regulator and corruption watchdog AUSTRAC.