Australian authorities have carried out their largest “counter-terrorism” raids so far, detaining 15 people to stop an alleged plot by supporters of the Islamic State of Iraq and the Levant (ISIL) to carry out “demonstration killings” in public.

A pre-dawn operation was carried out on Thursday across Sydney and Brisbane by more than 800 officers acting on some 25 search warrants.

One of those arrested was identified as Omarjan Azari, 22, who appeared briefly in a Sydney court on Thursday, according to AP news agency.

Prosecutor Michael Allnutt said he was involved in a “plan to commit extremely serious offences” that was “clearly designed to shock and horrify” the public.

Police have also issued an arrest warrant for Mohammad Ali Baryalei, 33, a former Sydney nightclub bouncer who is suspected to be Australia’s most senior member of ISIL.

The raids, which spanned multiple suburbs, came barely a week after Australia boosted the terror threat level to “high” for the first time in a decade, on growing concern about armed members of ISIL returning from fighting in Iraq and Syria.

“Police believe that this group that we have executed this operation on today had the intention and had started to carry out planning to commit violent acts here in Australia,” Andrew Colvin, federal police chief, said.

“Those violent acts particularly related to random acts against members of the public.”

This prompted comparisons to the murder of British soldier Lee Rigby, who was hacked to death in a random attack on a street in England last year by two Muslim converts.

Tony Abbott , Australian prime minister, said he had been briefed on intelligence that public executions had been ordered by ISIL.

“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 said.

“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.”

The Australian Broadcasting Corporation said court documents were expected to reveal that the plan involved snatching a random member of the public in Sydney, draping them in an ISIL flag and beheading them on camera.

Asked whether this was the case, Colvin replied: “That allegation will relate to serious violence on a random member of the public here on the streets of NSW [the state of New South Wales].

“Let’s let it run its course in court.”

The Australian government believes up to 60 Australians are fighting alongside ISIL, while another 100 were actively working to support the movement at home.

The latest raids followed the arrests of two people last week in Brisbane who were charged with allegedly recruiting, funding and sending self-declared jihadist fighters to Syria.

And, on Wednesday, a Sydney-based money transfer business was shut down amid concerns it was being used to funnel funds to the Middle East to finance “terrorism.” Al Jazeera