Police have disrupted a terrorist plot to detonate improvised explosive devices at locations in central Melbourne, possibly on Christmas Day, Prime Minister Malcolm Turnbull says

Victorian Premier Daniel Andrews described the plot as an "act of evil", while Victoria Police Chief Commissioner Graham Ashton said police had seized "the makings of an improvised explosive device".

Speaking at a press conference today, Mr Turnbull said: "Overnight our police and security agencies have disrupted a very substantial terrorist plot.

"Overnight, the Victoria Police, working with the Australian Federal Police and ASIO, have arrested seven persons, five of whom are still in custody, searched five premises and are continuing to search them.

"What they have uncovered is a plot to explode improvised explosive devices in central Melbourne in the area of Federation Square, on or about Christmas Day.

The raids were part of a counter-terrorism operation involving Victoria Police, the Australian Federal Police and ASIO.

Police believe the locations to be targeted include Federation Square, Flinders Street Station, and St Paul's Cathedral.

Commissioner Ashton said: "We believe [the plot] was going to involve an explosive event, the use of explosives, and we gathered evidence to support that.

"There has also been evidence that we will lead around the possibility of an intention to use other weapons. That could include knives and/or a firearm.

"Certainly these [people] are self-radicalised, we believe, but inspired by ISIS and ISIS propaganda."A 24-year-old man from Meadow Heights, a 26-year-old man from Dallas, a 22-year-old man from Campbellfield, a 21-year-old man from Flemington and a 21-year-old man from Gladstone Park remain in police custody.

A 20-year-old woman and a 26-year-old man, both from Meadow Heights, were released without charge.

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The five people in custody were expected to face court this afternoon, charged with acts in preparation of a terrorist event, Commissioner Ashton said.

Commissioner Ashton said four of the five were Australian-born with a Lebanese background.

"The age groups range between 20 and 24 or 25. There is another suspect in this matter who will be charged that was an Egyptian-born Australian citizen. All the others were Australian-born," he said.

Premier Daniel Andrews said there would be an increased police presence at large gatherings in Victoria over the Christmas period.

Commissioner Ashton said he spoke to Cricket Australia about providing additional security at the MCG during the Boxing Day Test, which starts on Monday.

(Australian Broadcasting Corporation)