A man alleged to have fallen asleep next to his gun had a rather terrifying wake-up call when his car was surrounded by heavily armed police in a suburban car park.

Police were called to a car park behind an IGA supermarket on Brice Avenue in Mooroolbark after receiving reports of a man slumped in a silver Ford Falcon with a gun resting on the front passenger seat about 7.40am on Thursday.

A man known only as Brett, who raised the alarm, told radio station 3AW he saw the man slumped over the wheel and then spotted the gun.

"There was a gun in the front seat," Brett told 3AW.