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(CNN) -- Australian authorities disrupted what they described as a pending attack "on a member of the public," part of a sweeping counterterrorism operation on Thursday that came just days after the country raised its terror alert level to high.
At least one person was arrested in connection with "terrorism offenses" and another 15 were detained during more than two dozen searches in Sydney, acting Federal Police Commissioner Andrew Colvin said at a news conference.
Authorities declined to detail the specific threat, citing an ongoing operation. The identity of the suspect in custody and the specific charges were expected to be released later in the day following a court appearance, they said.
However, Prime Minister Tony Abbott told reporters that authorities received intelligence that Islamic militants inside Australia had been urged to carry out a public execution or "demonstration killing."
"The exhortations, quite direct exhortations, were coming from an Australian who is apparently quite senior in ISIL (Islamic State) to networks of support back in Australia to conduct demonstration killings here in this country," he said.
Australia media reported that the suspects were planning to kidnap and behead a member of the public, before draping them in an ISIS flag.
More than 800 law enforcement officials fanned out across Sydney suburbs as part of a coordinated operation to serve 25 search warrants.
"Today's operation reflects the reality of the threat we actually face," Andrew Scipione, commissioner of New South Wales Police, said.
A second series of searches were carried out in Brisbane, Scipione said. Those searches were connected to the arrests earlier this month of two men in Brisbane accused of recruiting and financing foreign fighters to Syria.
"We are looking for linkages" between the Sydney operation and the one in Brisbane, Scipione said.
READ: Suspected Australian militant offers bounty Australia last week raised its terror alert level to "high" for the first time since the national alert system was introduced in 2003. It's the second-highest alert level.
According to the four-level system, a reading of "high" means a terrorist attack "is likely." The decision to raise the level was foreshadowed by the departing director-general of the Australian Security Intelligence Organization (ASIO) last week.
David Irvine said the threat level had been building in Australia over the last year, due to the increasing influence of jihadists fighting for Islamic extremists ISIS in the Middle East who had recruited Australians to fight.
READ: Who is doing what in the coalition battle against ISIS? This week, the Australian government was one of 40 nations that agreed to contribute to the fight against ISIS, the terror group that calls itself the Islamic State.
The government has agreed to send military advisers to Iraq, and it will deploy a number of aircraft, including fighter jets, to the United Arab Emirates. It also will also help to stem the humanitarian crisis. Australian combat troops will not participate in ground fighting, according to Abbott. CNN's Susanna Capelouto contributed to this report.