Australian counterterrorism forces detained 15 people Thursday in a series of suburban raids after receiving intelligence that the Islamic State militant group was planning public beheadings in two Australian cities to demonstrate its reach.

About 800 federal and state police officers raided more than a dozen properties across 12 Sydney suburbs as part of the operation -- the largest in Australian history, Australian Federal Police Deputy Commissioner Andrew Colvin told the Associated Press. A sword was removed as part of evidence at one of the homes.

Separate raids in the eastern cities of Brisbane and Logan were also conducted.

The Australian Broadcasting Corporation reported that the plan involved kidnapping randomly selected members of the public off the streets in Sydney and Brisbane, beheading them on camera, and releasing the recordings through Islamic State's propaganda arm in the Middle East.

Later Thursday, 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.

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

Prime Minister Tony Abbott and Brandis did not name the Australian. But Mohammad Ali Baryalei, who is believed to be Australia's most senior member of the Islamic State group, was named as a co-conspirator in court documents filed Thursday. Police have issued an arrest warrant for Baryalei, a 33-year-old former Sydney nightclub bouncer.

A 22-year-old Sydney man, Omarjan Azari, appeared in court Thursday and is accused of conspiring with Baryalei and others to act in preparation for or plan a terrorist act or acts.

Prosecutor Michael Allnutt said he was involved in a "plan to commit extremely serious offenses" that was "clearly designed to shock and horrify" the public. It is not immediately clear what sentence Azari faces if convicted. The accused did not apply for bail and did not enter a plea. His next court appearance was set for November 13.

Azari’s 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.

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.

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

Nine of those detained were later released, New South Wales police said.

A crowd of around 300 people protested the raids at a rally in Sydney on Thursday night.

Uthman Badar, a spokesman for the Islamist group Hizt ut-Tahrir, warned of a growing unrest within Australia’s Muslim community.

“We are tired of being made scapegoats. The government is the terrorist,” he said in front of supporters wearing anti-government placards, according to News.com.au.

“We would be fools to think we can now wake up and feel safer,” he added. “We are not fools to be deceived. There is anger in the community. We have been victimized for years and years.”

Meanwhile, Abbott told reporters that he had been briefed on Wednesday night about the operation and discussed the planned beheadings.

"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."

The planned violence resembled the murder of Lee Rigby, a British soldier who was attacked and killed in May 2013 by two Nigerian-born Muslim converts near the Royal Artillery Barracks in southeast London.

"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 added.

The arrests come just days after the country raised its terror warning to the second-highest level in response to the domestic threat posed by supporters of ISIS.

"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," Colvin said. "Those violent acts particularly related to random acts against members of the public. So what we saw today and the operation that continues was very much about police disrupting the potential for violence against the Australian community at the earliest possible opportunity."

New South Wales Police Commissioner Andrew Scipione said right now is a “time for calm.”

"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."

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. Colvin said the raids conducted in Brisbane on Thursday were a follow-up to that operation. It was not yet clear how the investigations in Sydney and Brisbane were linked, he said.

However, Fairfax Media reported that the arrests of the men averted a terror attack by mere days.

The government raised its terrorism threat last week from "medium" to "high" on a four-tier scale on the advice of the Australian Security Intelligence Organization. The domestic spy agency's Director-General David Irvine said the threat had been rising over the past year, mainly due to Australians joining ISIS to fight in Syria and Iraq.

When announcing the elevated threat level, Abbott stressed that there was no information suggesting a terror attack was imminent.

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

Australia has estimated about 60 of its citizens are fighting for ISIS and the Nusra Front in Iraq and Syria. Another 15 Australian fighters had 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.

The Associated Press contributed to this report.

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