The assault took place on Ashton Road, Harehills, at about 4.30pm on Wednesday.
Detectives say the caller reported seeing a man being attacked by a group of other men in the street.
But officers have been unable to trace the victim, and are appealing for the public’s help to identify him.
Detective Inspector Amanda Wimbles, of Leeds District CID, said: “We have been making a number of enquiries since this incident was reported to us, but so far have not been able to identify who the victim is and make checks on his welfare.
“I would urge anyone who knows who this man is, or indeed the victim himself, to please make contact with us.”
Anyone with information is asked to call Leeds District CID via 101, quoting crime reference 13180380653, or use the 101 Live Chat facility on the West Yorkshire Police website.
Information can also be passed on, anonymously, via independent charity Crimestoppers on 0800 555 111.