Two men were charged Thursday in connection with a terror plot that Australia's prime minister said involved plans to carry out a public execution.

Prime Minister Tony Abbott said authorities obtained intelligence about a "demonstration killing" -- a killing, Australian media reported, in which alleged assailants planned to kidnap a member of the public, behead the victim and then drape him or her in an ISIS flag.

The foiled plot comes just days after the country raised its terror alert to high and in the midst of Australia committing to helping the United States and a broad international coalition to beat back the Islamic State, or ISIS.

The militant group has been slaughtering people and taking over villages in Syria and Iraq. One of its members beheaded two Western journalists and an aid worker -- brutality that was videotaped and broadcast throughout the world. ISIS vows that it will stop at nothing to create a caliphate governed by the intensely strict Sharia law.

Earlier Thursday, at least one person had been arrested and charged with terrorism offenses and another 14 were detained as police conducted more than two dozen searches in Sydney, acting Federal Police Commissioner Andrew Colvin said at a news conference.

Hours later, police said nine people who were detained have been released.

A 24-year-old man from Merrylands has been charged with possession of ammunition without a license and unauthorized possession of a prohibited weapon, authorities said. Another man has also stood in court, charged with conspiracy to commit acts in preparation of a terrorist act and financing terrorism, police said, and two women were issued notices to appear later in court. Their names have not been released.

Authorities declined to give details about the threat, citing an ongoing operation.

But Abbott told reporters that intelligence revealed that Islamic militants inside Australia were planning to kill.

"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.

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," said Andrew Scipione, commissioner of New South Wales Police.

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.

The commissioner said authorities are looking for links between the Sydney operation and the one in Brisbane.

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Meanwhile, police presence has been increased at Parliament because of security threats, Abbott told Channel 7 on Friday.

Citing an intelligence review carried out in recent days, Abbott said federal police will be taking over security inside and outside of Parliament.

(CNN)