A teenager has been flown to Southampton Hospital for treatment after being assaulted twice in the early hours of Sunday morning ( 10 July).
The 19 year old man suffered serious head injuries.
Three men aged 21 and 22 have been arrested on suspicion of grave and criminal assault. They have been released on police bail.
Officers are appealing for witnesses to two incidents.
The first was outside Rojos on Beresford Street at around 2.20am, and the other was outside William Hill on Bath street a short time later, where the victim was assaulted for a second time and knocked unconscious.
He has been transferred to hospital in the UK.
"Anyone who may have seen this or who has any information is asked to contact police on 612612 or the independent charity Crimestoppers anonymously on 0800 555 111 or via their Anonymous Online Form at Crimestoppers-uk.org"