It happened at St Nicholas Street, Scarborough, around 1am, just as the clocks went from 2am, on Sunday and involved two men, one aged in their forties and one in their mid-twenties, who were involved in a discussion near to Quids Inn and Blue Lounge.
A North Yorkshire Police spokesman said: "Both men were then seen to go to the covered entrance to the council car park between the Town Hall and Scarborough Flyer.
"One man received serious head injuries and was taken to Hull Royal Infirmary where he is currently receiving treatment. His condition is described as stable.
"A man in his mid-twenties was arrested a short time later and has been released on bail pending further enquiries.
"We are conducting enquiries and are requesting the public’s assistance to help determine the full circumstances surrounding the incident.
"As there were many people out on St Nicholas Street during the night at Hallowe’en events, it is believed that people have witnessed the build-up and the assault.
"In particular, we are appealing for information from two men who immediately rushed to provide emergency first aid to the victim, and five women that were walking passed the incident on their way into town from the direction of the Royal Hotel.
"Anyone with information that could assist the investigation should contact us on 101, select option 2, and ask for Jon Kenworthy. You can also email [email protected]
"If you wish to remain anonymous, you can pass information to Crimestoppers on 0800 555 111.
"Please quote the North Yorkshire Police reference number 12160196913."