A police officer was hospitalised with a fractured eye socket after an unprovoked attack.
Three officers were helping an elderly man who had collapsed on a bus when they were assaulted at around 11pm on Monday, the Metropolitan Police said.
The sergeant and two constables were kicked and punched by a man who approached them in Alexandra Avenue, Harrow, north London.
One officer was taken to hospital after the attack but the other two were not seriously injured, the force said.
The three officers arrested a 36-year-old man on suspicion of grievous bodily harm and assault on police officers.
He remains in custody.
Chief Inspector Alison Darke said: “Whilst helping an elderly man and his grandson, three of our officers were subjected to a vicious and unprovoked attack which has resulted in one of them being hospitalised overnight.
“Policing is by its very nature unpredictable - however no officer should be subject to such a violent attack.”