Police have arrested one man after raids by Victoria's Counter Terrorism Team and Australian Federal Police in Melbourne's north-west and Tatura, near Shepparton.
Other warrants were executed across regional Victoria, including in Bacchus Marsh.
A 31-year-old man, believed to be a notorious right-wing extremist, was arrested by Special Operations Group members at an address in Ballarat Road, Braybrook about 1pm.
It is understood the man is Phillip Galea, a member of the anti-immigration group Reclaim Australia, who served jail time last year for possessing tasers and mercury.
Police escorted Mr Galea from the Ballarat Road property shortly before 4pm.
Victoria police counter terrorism command Assistant Commissioner Ross Guenther said that earlier this year they had received information that suggested there were individuals or an individual looking at either advocating harm against Victorian individuals or producing documents that might lead to the harm of people in Victoria.
"I'm very pleased to say I think we've interrupted something that could have been quite serious in terms of harm to our community," he said.
Assistant Commissioner Guenther said he was not in a position to confirm the location of the terror threat.
"I'm satisfied that the threat more broadly would be in Victoria," he said.
"I can say that the threat was specific enough to cause us alarm in terms of what the risk represented to the community. So whether or not that would be something that put one person at risk or several people at risk, I can't say at this point in time."
Assistant Commissioner Guenther would not comment on what police had seized, after four Victorian properties were searched on Saturday.
"I'm sure that will be coming out over the next 24 to 48 hours," he said.
He said the Braybrook man who is in police custody was a "person of primary concern" and over the next 24 hours there may be more people implicated and more properties searched.
When asked whether the man was affiliated with Reclaim Australia, he said "I'm not in a position to confirm that but I can say that the person in custody has a number of different affiliations to organisations".
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