A man has died in police custody shortly after he was handcuffed by officers at a bar in Melbourne's south-east this morning.

A post-mortem examination is underway to determine the exact cause of death of the man, who was arrested at a bar on Glenhuntly Road in Elsternwick around 2:15am.

Police were called to the bar following reports the man had assaulted other patrons.

In a statement, Victoria Police said the man was already being held down by "a number of people" when police arrived.

Police said officers then arrested the man and placed him in handcuffs "when he lost consciousness".

"Police attempted to revive the man until Ambulance Victoria arrived and took over," the statement said.

"Despite the efforts of emergency service personnel, the man died at the scene."

The homicide squad is investigating the death with oversight from Professional Standards Command, which is the protocol for all Victorian deaths in custody.

The man has not yet been formally identified.