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.
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.