The Victoria Police bomb response unit has been called to a property at Epping, in Melbourne's north, after police investigating bikie gang wars arrested four men.
Police say a suspicious device has been found at one of the properties raided this morning.
The police taskforce called Operation Resound executed a number of warrants and raided properties in a number of suburbs including Epping, Whittlesea and Wollert.
Four men are in custody at the crime squad headquarters on St Kilda Road in Melbourne.
Police say two stolen jet skis have been seized, as well as a stolen motor vehicle and a number of firearms.
A hydroponic set-up located at a property in Craigieburn has also been seized.
Assistant Commissioner Steve Fontana says more raids will be carried out throughout the day.
"We've got more warrants to be executed," he said.
"On Monday night we executed a warrant at the Nomads chapter of the Hells Angels, we seized a loaded sawn-off shotgun and drugs and other type of paraphernalia."
The arrests come as police e escalating between the Bandidos and Hells Angels outlaw motorcycle gangs.
It follows a second attempt on the life of Bandidos Sergeant At Arms, Toby Mitchell last Friday.