Police believe four men were involved in a fight on Roundhay Road at Harehills on Monday afternoon (Nov 19) when a 33-year-old man was stabbed in the arm.
A 38-year-old man who was working nearby tried to break up the fight.
He was punched and knocked unconscious during the incident, which happened On Roundhay Road outside a parade of shops near :Lambton Grove.
Police said that just before 3.30pm on Monday officers received a report that 33-year-old man had attended hospital with a serious stab wound to his arm.
He was arrested in connection with the incident and is continuing to receive treatment.
His injuries are not considered life threatening.
Detective Inspector Phil Jackson, of Leeds District CID, said: “Our enquiries into this incident are still at a relatively early stage but it appears there has been an altercation in the street between two men who arrived in a car and two other men who were stood in the street.
"This has led to them fighting with each other during which the 33-year-old man was stabbed in the arm.
“The 38-year-old man was working nearby and intervened during the fight and was punched and knocked unconscious.
"He was treated at hospital and found to have not been seriously injured.
“The other men involved have left the scene and we are continuing enquiries today to identify all those involved.
“This happened in a busy part of Harehills during daylight and will have been witnessed by a number of people.
"We would like to hear from anyone who has any information that could assist the investigation.
“Violent incidents such as this cause understandable concern in the local community and will not be tolerated and we will do everything we can to identify all those responsible and deal with them appropriately.
Anyone with any information is asked to contact Leeds District CID quoting crime reference 13180581169 or call the independent charity Crimestoppers anonymously on 0800 555 111.