As many as 65 handguns have been stolen in an armed robbery by masked thieves in Melbourne's north, with police fearing the weapons could be used by criminal groups.

Four masked thieves armed with weapons, including a gun and sledgehammer, raided O’Reilly’s Firearms on High Street, Thornbury at 11am on Monday.

Speaking on radio 3AW on Tuesday morning, Assistant Commissioner Steve Fontana said up to 65 guns were taken in the raid.

"Basically they'll end up on the black market in the hands of criminals," Mr Fontana said.