At least two young men from Melbourne's south-eastern suburbs allegedly planned to kill a police officer and use the officer's gun to go on a shooting rampage in an Islamic State-inspired attack, it has been reported.

Seven News reported that the alleged attack was due to be "shockingly similar" to the 2013 murder of British soldier Lee Rigby in south-east London.

Police believe the alleged terror plot was partly payback for the death of teenager Abdul Numan Haider, who was shot dead last September after stabbing two anti-terror police officers in Melbourne.

Fairfax Media understands that some of the young men arrested in pre-dawn counter-terrorism raids on Saturday were with Haider before he attacked two officers with a knife outside the Endeavour Hills police station.

The alleged plotters had discussed opening fire at one or more Anzac Day events in south-east Melbourne suburbs, possibly at a local RSL march, Fairfax Media has been told.

It allegedly involved running a police officer over and then killing him with a knife, the ABC's 7.30 program reported on Monday night.

Those behind the plot then allegedly planned to take the officer's gun and go on a shooting rampage that would have ended in their deaths, it was reported.

Police revealed on Monday night that a 14-year-old boy had been arrested in Britain over his links to the plan. Lancashire police have claimed they uncovered messages on electronic devices between the teenager and a man in Australia.

Counter terrorism police from the United Kingdom have flown to Melbourne to assist local investigators following Saturday's raids.

Two officers, a detective chief inspector and a detective sergeant, from the North West Counter Terrorism Unit based in Manchester are in Melbourne, Victoria Police spokeswoman Sharon Darcy said.

Sergeant Darcy said the investigators are assisting Victorian and federal police in the Joint Counter Terrorism Team.

Five men were arrested by Victoria Police in the pre-dawn counter-terrorism raids on Saturday.

One of the men – Harun Causevic from Hampton Park - was held in custody on a Preventative Detention Order – the first time such an order has been used in Victoria.

The 18-year-old was charged with conspiracy to commit acts done in preparation for, or planning, terrorist acts on Monday night. The detention order was formally lifted in the Supreme Court on Tuesday morning.

Mr Causevic appeared briefly in Melbourne Magistrates Court on Tuesday, dressed in a black T-shirt and with his shoulder-length brown hair untied.

The teenager did not say anything during his appearance, but spent a few moments looking at people in the packed court room, in which some of his family members sat. A group of Australian Federal Police officers, wearing Anzac Day pins on their suit jackets, were also in court.

The court was told Mr Causevic had been held since his arrest in the high-security Acacia unit in Barwon Prison, with some of Victoria's most serious convicted criminals. His defence lawyer Rob Stary said his client should be held in a secure unit given his age and circumstances until he applied for bail on April 30.

Outside court Mr Causevic's father, Vehid Causevic, spoke of a "set-up".

"That is all a set-up," he said.

Sevdet Besim, 18, was the first man to be charged following the raids. Mr Besim, from Hallam, was charged on Saturday with conspiring to commit an act of terror. He has been remanded and is due to face court on Friday.

A charge sheet tendered to court alleges that Mr Causevic and Mr Besim prepared, or planned, a terrorist act between February 23 and April 18.

An 18-year-old man, from Narre Warren, was arrested and charged with prohibited weapons offences. He has been granted bail and will face court at a later date.

Another 18-year-old Narre Warren man was released pending further inquiries.

Eathan Cruse, 19, who was also arrested, gave a statement to police and was released without charge on Saturday afternoon.

All five men are believed to have had close links to senior Australian jihadist Neil Prakash.

Fairfax Media understands that police are investigating the role in the plot played by Mr Prakash, 23, who is regarded by counter-terrorism authorities as the top Australian recruiter for Islamic State in Syria.

Mr Prakash, who goes by the jihadist name Abu Khalid al-Cambodi, may have had direct contact with the Melbourne men recently, forming a link between the alleged plot and the terror group causing upheaval in the Middle East.

The men arrested on Saturday have also been linked to the Al-Furqan Islamic Information Centre in Springvale South, where leaders are suspected of preaching radical ideas.

Mr Haider also attended the Al-Furqan Centre.

Hundreds of extra police will be present at Anzac Day ceremonies across Victoria on Saturday, police said. There will also be secret security operations under way at the weekend.

Acting Police Commissioner Tim Cartwright said Melbourne's dawn service would be a key focus for officers, but other events would also be closely monitored.

"We are not restricting ourselves to the dawn service nor just to Melbourne and the Anzac Day events on that day," he said.

"We are looking right across Victoria for the events that relate to Anzac Day."

Premier Daniel Andrews reassured people heading to events, including the football at the MCG, that police were working to ensure their safety.

"Whether it be at the Shrine, other events that will occur in the suburbs and regional centres and indeed at the MCG, the rugby that evening," he said.

"I'm very confident that everything that can be done, is being done to keep Victorians safe."

Five arrested, three charged

* Sevdet Besim, 18, from Hallam: Charged on Saturday with conspiring in the preparation for, or planning of, a terrorist act. Remanded in custody and due to return to court on Friday.

* Harun Causevic, 18, from Hampton Park: Charged on Monday with conspiring in the preparation for, or planning of, a terrorist act. Remanded in custody and due to return to court on April 30.

* An 18-year-old man from Narre Warren: Charged on summons on Saturday with the possession of prohibited weapons. On bail and due to face court at a later date.

* An 18-year-old from Narre Warren: Released from custody on Saturday pending further inquiries.

* Eathan Cruse, 19, from Narre Warren: Gave a statement to police on Saturday and released from custody.