Hundreds of Australian police conducted raids in a pre-dawn counter-terrorism operation against Islamic State militants who were allegedly planning a public beheading of a random citizen, Prime Minister Tony Abbott said Thursday.

About 800 police officers carried out the operation, resulting in the detention of 15 people. The operation was the largest in Australian history, police said, and focused on suburbs in western Sydney and Brisbane.

According to Abbott, the radicalized citizens had planned to conduct and record a beheading on ISIS' behalf:

That's the intelligence we received... 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.

One of the men, 22-year-old Omarjan Azari of Sydney, has been charged with conspiracy to commit a terrorist act.

"What we saw today in the operation that continues was very much about police disrupting the potential for violence against the Australian community at the earliest opportunity," Acting Comm. Andrew Colvin of the Australian Federal Police said in a press conference Thursday.

Australia had raised its national terror threat level to "high" for the first time earlier this week out of concern over Australians radicalized in Iraq or Syria carrying out attacks in the country. In July, an Australian citizen conducted a suicide bombing that killed three people in Baghdad. Two others were found to have posted images on social media holding severed heads of Syrian soldiers.

Reuters reported that many residents of the largely Muslim neighborhood did not hear about the raids and were skeptical of the terrorism plot, especially as the actions could worsen relations between authorities and the public.

"I hope they have very solid facts," Samier Dandan, president of the Lebanese Muslim Association, told Reuters, "because if they don't, this is going to be the basic platform from which the community engages with law enforcement moving forward from here."

According to the Sydney Morning Herald, officials have estimated up to 160 Australians have become involved in the fighting in the Middle East or are actively supporting it, with at least 20 who have returned to Australia, posing a security risk.