Three men arrested on suspicion of plotting a terror attack in Melbourne had tried to get semi-automatic rifles to kill as many people as possible in a crowded space, police allege.

The men, aged 30, 26 and 21, were from the city’s northern suburbs. Victoria Police Chief Commissioner Graham Ashton said police had been monitoring the trio since March.

Commissioner Ashton said the men, Australian citizens of Turkish descent, were arrested on Tuesday morning during counter-terrorism raids as part of Operation Donabate by Victoria Police, the Australian Federal Police and ASIO.

Police will allege the three men arrested were involved in “undertaking preparations for planning a terrorist act” in a crowded place “where maximum people would be attending”.

The three men, two of whom are believed to be brothers, remain in custody and the counter-terrorism operation is ongoing.

Assistant Commissioner Ross Guenther of Victoria’s Counter Terrorism Command said it was alleged the three men were trying to obtain semi-automatic rifles prior to their arrest.

“They’ve been of interest to us based on some information we received early last year in 2017,” Assistant Commissioner Guenther said.

“We commenced an investigation on the basis of that information … When that information caused us enough concern, we made a decision to pass that investigation across to the Joint Counter-Terrorism Team.”

AFP Assistant Commissioner Ian McCartney said the three men had their passports cancelled earlier this year after concerns they were planning to “be involved in terrorist activity offshore”.

Assistant Commissioner McCartney said there was no evidence the potential attack was directed by Islamic State.

“It’s an attack inspired by ISIS, driven by the propaganda machine and unfortunately people are still listening to that propaganda machine in terms of what they espouse,” he said.

Officers executed four warrants during the raids, which also involved the search of a fourth property at Coolaroo.

“Victoria Police would like to reassure the community that the threat has been contained and there is no ongoing threat.” 

Victorians were urged to go about their daily business, but remain vigilant and report any suspicious activity to Triple Zero, Crime Stoppers (1800 333 000) or the National Security Hotline (1800 123 400).