A 14-year-old girl has been arrested on suspicion of punching an 87-year-old woman in the face on a bus.
The victim suffered a black eye and needed hospital treatment after what police called a "nasty attack" in Croydon, south London.
Earlier, police released CCTV footage of the assault.
Two teenage girls had boarded the route 166 without paying and then began arguing with the driver - prompting most of the passengers to leave the bus.
Police said the victim, "who just wanted to get home", was standing next to the pair when she was struck in the attack at around 3.20pm on 16 October.
Both girls, described as black and aged between 14 and 18, ran away from the vehicle in Brighton Road, Coulsdon.
The younger girl was held on suspicion of actual bodily harm.
A passenger came to the aid of the victim, who was taken to hospital and discharged the same day.
Stills from the bus CCTV of the suspected perpetrators