CCTV images of three women and a man have been released and they are being urged to come forward with any information they might have on the attack.
Paramedics were alerted to reports that a man had been found lying unconscious outside the Dog Inn pub in Church Street at around 4.30am on Thursday December 20.
Ambulance crews attended to the man and informed police that the man's head injury was likely the result of an assault.
He was taken by ambulance to Royal Preston Hospital where he remains in a serious but stable condition.
Following an initial CCTV appeal for information, a 53-year-old man was arrested on suspicion of assault but was released under investigation pending further enquiries.
A subsequent appeal to trace a man in a silver jacket resulted in a second arrest of a 22-year-old man from Preston on suspicion of assault.
He has also been released under investigation pending further enquiries.
Police now want to speak to the people pictured as they believe they may be witnesses.
If you recognise yourself or perhaps someone you know in the images, you can call police on 01772 209745 or email [email protected]
If you fail to get an answer, please dial 101 and quote log number 0142 of December 20.
Alternatively, you can reach independent charity Crimestoppers anonymously on 0800 555111.