Australian police have charged a teenager with conspiring to commit a terrorist attack at an ANZAC Day event next week.
The attack was planned to take place at an event to mark the centenary of the landings at Gallipoli during World War 1.
Ceremonies are held across Australia to remember fallen Australian and New Zealand Army Corps troops.
Prime Minister Tony Abbott said hundreds of Australian police were involved in an operation which resulted in the arrest of five teenagers in an overnight raid.
Sevdet Besim, 18, from the southeast of Melbourne was charged with conspiring to commit a terrorist act on ANZAC Day on 25 April.
He was refused bail and is expected to appear in court next Friday, Australian media reported.
An unidentified second 18-year-old who was arrested but not yet charged was being held under a preventative detention order that could last for up to two weeks, a Victoria Police spokeswoman said.
It is believed to be the first time a preventative detention order - meant to help police when there is a threat of an imminent terrorist attack or to collect evidence after an attack - has been used by the southern state's police force, the spokeswoman said.
A third man, also 18, was released and is expected to be charged with weapons offences, while two others aged 18 and 19 were freed without charge as investigations continued.
The run-up to this year's centenary, a major holiday in Australia and New Zealand, has been marked by numerous television programmes and commemorations.
It has sparked concerns that radicals may target the celebrations for a high-profile attack.
Australia has sent hundreds of soldiers to Iraq to help train forces fighting the self-styled Islamic State, heightening concerns about reprisal attacks in the homeland.
Australia, a staunch ally of the US and its action against the Islamic State group in Syria and Iraq, has been on heightened alert for attacks by home-grown Islamist radicals since last year.
Canberra raised the national terror threat level to "high"; for the first time last September.
Hundreds of police conducted raids after receiving information that so-called Islamic State supporters planned to conduct a public beheading.
Australia believes at least 70 of its citizens are fighting with the so-called Islamic State group in Syria and Iraq, backed by about 100 Australia-based facilitators.