Hundreds of Australian police struck in dawn raids in Sydney and Brisbane as intelligence services moved to forestall what they believe was an imminent terrorist attack.

Fifteen people were arrested on suspicion of preparing a public execution to further the cause of Islamic militants in Iraq and Syria. Police believe the plan was to snatch someone from the streets of Sydney, and then kill them on camera in front of an ISIL flag. The prime minister says the attack was designed to terrify people.

“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, 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,” said Tony Abbott.

Officials say some 160 Muslim Australians are believed to have been involved in fighting in the middle east, and some 20 have returned. Australia has about half a million Muslims, half of whom live in Sydney’s western suburbs. Australia hosts the G20 summit in November.