SYDNEY (AP) — Police on Thursday said they thwarted a plot to carry out beheadings in Australia by supporters of the radical Islamic State group. They detained 15 people and raided more than a dozen properties across Sydney, though nine of those brought in were freed before the day was over.

The raids involving 800 federal and state police officers — the largest in the country's history — came in response to intelligence that an Islamic State group leader in the Middle East was calling on Australian supporters to kill, Prime Minister Tony Abbott said.

Abbott was asked about reports that the detainees were planning to behead a random person in Sydney.

"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."

ISIL refers to the al-Qaida splinter group leading Sunni militants in Iraq, Islamic State of Iraq and the Levant, which now calls itself simply Islamic State.

"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.

Nine of those detained were later released, New South Wales police said. They did not say why, or whether they will 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.

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.

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

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.

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.

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, whom police didn't name, was charged with possessing ammunition without a license 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 had been rising over the past year, mainly due to Australians joining the Islamic State movement to fight in Syria and Iraq.

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-Qaida 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.

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 about 60 of its citizens are fighting for the Islamic State group 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.

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) a month to the Middle East to finance terrorism, said John Schimdt, chief executive of the industry regulator and corruption watchdog AUSTRAC.

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Associated Press writer Rod McGuirk in Canberra, Australia, contributed to this report.