A man was reportedly beaten with a baseball bat during a fight on a busy road in east London.
Police have released CCTV of the alleged victim after the incident in Eastern Avenue, Ilford, at 3.30pm on Saturday.
Officers are concerned that he may have sustained a head injury, and are urging him to make contact with police and to attend hospital if the injury has not yet been treated.
There is no record of him attending hospital, Scotland Yard said.
Emergency services rushed to the scene of the incident and found a 36-year-old man, who police believe was involved in a fight with a second man.
The 36-year-old man has been was arrested on suspicion of GBH and taken to an east London police station, where he remains in custody.
Having spoken with witnesses at the scene, officers wish to trace the second man, who they believe was struck with a baseball bat and left the area before police arrived.
They are appealing for anyone who can identify the man - or for the victim himself - to contact them urgently.
Anyone who can assist police should call East Area CID on 07979 311259.