Police were called around 2.35am on Saturday December 28 to reports two men were fighting at the junction of Marine Road Central and Queen Street.
A 38-year-old man was found at the scene with a broken leg and head injury.
He was taken to Royal Lancaster Infirmary for treatment.
Another man was arrested nearby in connection with the incident.
Detectives are now appealing for information and are keen to speak to any witnesses.
Det Con Peter Bennett, of Lancaster CID, said: “We are keen for any information in relation to this incident.
“A man has been left with a serious leg injury and we are keen to establish what has happened.
“If you saw what happened, or can assist with our enquiries, please come forward.”
Anyone with information can contact police on (01524) 596452 or email [email protected] quoting log 0132 of December 28.
Alternatively Crimestoppers can be contacted anonymously on 0800 555 111.
A 37-year-old man from Morecambe was arrested on suspicion of assault and later released under investigation.