Increased Security after Planned Australia Anzac Day Attack

Security will be boosted at Anzac Day commemorations in Australia's state of Victoria, as its premier on Sunday urged people to turn up despite arrests linked to an alleged Islamic State-inspired attack plan.

Police arrested five men in counter-terrorism raids in the state's capital Melbourne on Saturday and charged one of them -- 18-year-old Sevdet Besim -- with conspiring to commit a terrorist act on Anzac Day on April 25 when ceremonies are held across the nation to remember fallen Australian and New Zealand Army Corps troops.

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 told Agence France Presse Sunday.

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 Australian 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.

Premier Daniel Andrews said he hoped "as many Victorians as possible will be at the Shrine of Remembrance and other Anzac Day services right across our state", adding that his family would be attending the events.

"The Victorian community can rest assured that Victoria Police in co-operation with the Australian Federal Police and other security agencies will have in place on Anzac Day the best possible arrangements to do everything that can be done to keep Victorians safe."

There will be an increased police presence at some sites, including at the Shrine of Remembrance in Melbourne where thousands of people are expected to attend an Anzac Day dawn service.

Prime Minister Tony Abbott on Saturday said he expected security to be ramped up at other Anzac Day events.

He urged Australians to attend the ceremonies "in the largest possible numbers", a call echoed by Labor opposition leader Bill Shorten.

"I would encourage people not to be deterred by these raids yesterday, but rather demonstrate together, side by side, what we can do and that we will not be deterred by a fear of terrorism," Shorten said Sunday.

This year's events have assumed added significance as the day marks a century since the bloody World War I Gallipoli campaign in what is now Turkey.

Some 11,500 of the 60,000 Australian and New Zealand troops who joined the 1915 campaign died.

Australia has warned of the threat from "home-grown" Islamic State-inspired extremists and has unveiled new security measures including revoking citizenship for dual-nationals linked to terrorism.

Canberra raised its threat level to high in September and carried out a series of counter-terrorism raids last year.

In December, a siege in a Sydney cafe by a self-styled cleric left the gunman and two hostages dead.