A 30-year-old man is in Royal Bournemouth Hospital with serious head injuries.
Police said they were called to an altercation outside Chilli White on St Peter's Road in Bournemouth town centre at 3.16am.
A 20-year-old man Bournemouth man has been arrested.
Detective Inspector Becky Riggs said: "There were lots of people in the area and I would encourage anyone with information to contact Dorset Police urgently.
"This incident left a man with a serious injury and it's vital we establish exactly what happened."
The injured man as said to be in a serious but stable condition.
Police did not say where he was from.
You can contact Dorset Police on 101 about incident 78 of February 4, or call Crimestoppers on 0800 555 111.
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