Australian police will allege that members of a group targeted in a sweeping counter-terrorism operation on Thursday planned to behead a random member of the public after draping the victim in the flag of Islamic State militants, Australia media reported.
Without referring to specifics, Prime Minister Tony Abbott said Australia was at "serious risk from a terrorist attack."
He told reporters that the large-scale counter-terrorism raids in Sydney and Brisbane followed intelligence that Islamic militants were urging supporters to conduct "demonstration killings" in Australia.
Court documents to be revealed later on Thursday were expected to show the plan involved snatching someone in Sydney and executing them on camera, the Australian Broadcasting Corporation and other Australian media said.
More than 800 police were involved in the pre-dawn raids, described as the largest in Australian history. At least 15 people had been detained, with one man charged with a serious "terrorism-related" offence, police told a news conference.