Two gunmen are on the run after they fired shots, assaulted a man with a rifle and stole cash from a gaming venue and bar in Melbourne's southeast.

The armed men stormed the Vegas at Waverley Gardens venue on Friday night demanding money.

A witness, who asked not to be identified, told AAP one of the men had pointed a gun at him and his friend was hit in the head by the butt of a rifle.

"He ran past and smashed my friend in the head with the rifle butt," the man told AAP.

A man in his 50s was taken to Dandenong Hospital with upper body injuries, an Ambulance Victoria spokeswoman said.

The two gunmen ran out of the bar and fired a shot in a nearby undercover car park, Victoria Police have confirmed.

Dozens of police arrived at the scene but the men were able to flee with an "undetermined" amount of cash.