A 71-year-old man suffered "potentially life-changing" head injuries in an apparent unprovoked attack in Croydon town centre yesterday.
Police arrested a 29-year-old man, thought to have run out of an NHS mental health centre before the assault in Tamworth Road just before 2.10pm.
Police are expected to charge the attacker with causing grievous bodily harm later today.
It is understood he emerged from Tamworth Road Resource Centre, which treats adults with servere mental health needs, before the attack.
Transport for London (TfL) suspended trams between Reeves Corner and East Croydon for four hours following the attack, which took place on the tracks.
The 29-year-old remains in police custody and detectives are continuing to investigate the assault.
A Metropolitan Police spokesman said: "A 71-year-old man remains in hospital after being assaulted in Tamworth Road. He is in a stable condition but his injuries are potentially life-threatening.
"A 29-year-old man arrested at the scene remains in police custody.
"Detectives are continuing with their enquiries."
Did you see what happened? Call the newsdesk on 0208 722 6351 or email firstname.lastname@example.org