A man who tackled a man armed with a gun in a Melbourne shopping centre was "smashed in the head" with the weapon before the offenders fled.

Authorities were called to Waverley Gardens Shopping Centre in Mulgrave after 6.30pm.

It's believed two men, both armed with a firearm, entered a licensed premise on Police Road and demanded cash.

A major police operation is underway after reports of a shooting inside a Melbourne shopping centre. (9NEWS)

They then fled into the shopping centre car park while being chased by patrons. Authorities said one of the suspects then fired a shot into the roof.

One person was assaulted during the incident and taken to hospital with minor injuries.

The two men fled in an unknown direction.

A witness told the Herald Sun he thought a fight was breaking out in the bar.

“A guy I was having a drink with tackled this guy at the end of the stairwell and then I saw another one at the cash register who pointed a gun,” the witness said.

The other gunman then ran past and "smashed my friend in the head with the rifle butt," the man said.

The exact circumstances surrounding the incident are still being determined and no arrests have been made, police said.

Dandenong North resident Ben, who did not want to reveal his surname, told 9news.com.au he saw two men walking out of the shopping centre with rifles.

"My nephew and I were driving into the lower level car park, I saw these two guys come out of the doors, near a Pokies thing they've got there," he said.

"They were wearing hoodies.

"They were running and then about half a dozen members of the public came running out, they were shouting things like 'how dare you' while they chased after them.

"And then I hear a loud bang, I've never heard a gunshot in my life, I have no doubt it wasn't just a car backfiring.