CANBERRA, Australia (AP) -- Police in Australia have detained five men suspected of planning a series of Christmas Day bomb attacks in the heart of the country's second-largest city, officials said Friday.

The suspects had been inspired by the Islamic State group and planned attacks on Melbourne's Flinders Street train station, neighboring Federation Square and St. Paul's Cathedral, Victoria state Police Chief Commissioner Graham Ashton said.

The arrests came after a truck smashed into a Christmas market in Berlin on Monday, killing 12 people. A manhunt is underway for the person behind that attack, which prompted increases in security around the world.

Two of seven people initially arrested in raids Thursday night and Friday morning in Melbourne — a 26-year-old man and a 20-year-old woman — were released without being charged, police said.

Five men between the ages 21 and 26 remained in custody and would be charged later Friday with preparing a terrorist attack. They were not identified but police said four were born in Australia and the fifth was Egyptian-born with Egyptian and Australian citizenship.

Police had been watching the alleged plotters for some time, and believed they were preparing to use explosives, knives and a gun, Ashton said.

Police believed the threat had been neutralized through the raids on five Melbourne premises, he said.

Prime Minister Malcolm Turnbull said: "This is one of the most substantial terrorist plots that have been disrupted over the last several years."

"Islamist terrorism is a global challenge that affects us all. But we must not be cowed by the terrorists," Turnbull told reporter.

"We will continue to go about our lives as we always have. What these criminals seek to do is to kill. But they also seek to frighten us, to cow us into abandoning our Australian way of life. They want to frighten Australians. They want to divide Australians. They want us to turn on each other. We will not let them succeed," he added.

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Since Australia's terrorist threat level was elevated in September 2014, the government says there have been four extremist attacks and 12 plots foiled by police.

Australian Federal Police Commissioner Andrew Colvin said the plotters had moved very quickly from a plan to develop a capability to attack.

"In terms of events that we have seen over the past few years in Australia, this certainly concerns me more than any other event that I've seen," Colvin said.

"We believe that we have removed the bulk of this particular cell, this group," he said.

Victoria state Premier Daniel Andrews said there will be extra police on the streets of Melbourne on Christmas Day to make the public feel safe.

About 400 police officers were involved in the raids.

Ashton described those arrested as "self-radicalized" and inspired by Islamic State propaganda.