800 heavily-armed Australian police officers from Brisbane and Sydney carried out major anti-terror raids on Thursday after a call from an Islamic State militant warning of “demonstration killings.” The officers raided homes in the largest counter-terrorism operation in Australian history and detained 15 people on conspiracy charges.

The raids began after the police intercepted a phone call two days ago from Afghan-born 33-year-old Australian nightclub bouncer Mohammed Ali Baryalei, a senior Australian Islamic State militant, to 22-year-old Omarjan Azari who had been allegedly instructed to carry out an attack on random citizens. These members of the public would be kidnapped, draped in an Islamic State flag and then beheaded.

Prosecutor Michael Allnutt said that Mr. Azari was one of a number of people who had planned to “commit extremely serious offences” that were “designed to shock and horrify” the public. Mr. Azari will appear in court on November 13.

Australian Prime Minister Tony Abbott held a press conference where he stated that he had been briefed on the exhortations. “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 Mr. Abbott.