Police in Victoria have arrested four people over a wild brawl in Melbourne's CBD on Saturday.

Shocking video of an unprovoked coward punch in Melbourne’s CBD was shared online after the violence.

Four people have now been arrested - two for drunkeness offences, one for possessing a taser and another for assaulting police.

Footage captured during a brawl in Melbourne’s Swanston St, shows the terrifying moment a man is hit in the head from behind.

The victim, a man in navy blue, can be seen standing in the back of the group.

All of a sudden a man in red appears in the footage, he slowly approaches the man in navy from behind and delivers a punch in the back of his head.

The man in red then flees the scene as his victim falls to the ground, appearing to be unconscious.

The victim’s condition was unknown.

The Daily Mail reported paramedics were called to a brawl on Swanston St on Friday night about 11.50pm.

A person called an ambulance and reported a man had been assaulted and was lying on the ground.

It’s believed when paramedics arrived no patient was to be found.

The shocking punch footage was posted on Facebook, the post claimed the fight was outside McDonalds on Swanston St in Melbourne’s CBD.

Police said one man went to hospital and another two suffered minor injuries.

Police have called for anyone with further information to come forward.