DHAKA: Australian police have carried out major anti-terror raids triggered by a call from a senior Islamic State militant for "demonstration killings", Prime Minister Tony Abbott says.

The raids, involving at least 800 heavily-armed officers, took place in Brisbane and Sydney.

Fifteen people were arrested and one person charged with "serious terrorism-related offences", police said.

Police say the anti-terror operation was the largest in Australian history.

Addressing a press conference, Abbott said "direct exhortations were coming from an Australian who is apparently quite senior in ISIL (Islamic State) to networks of support back in Australia to conduct demonstration killings here in this country".

"So 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."

Reports in the Australian media said a plot involved beheading a random member of the public after draping them in an Islamic State flag, reports BBC.

In the raids, Australian police have made 15 arrests in Sydney and Brisbane, in what they say is the nation's biggest counter-terrorism operation.

BDST: 1053 HRS, SEP 18, 2014