A 14-year-old girl has been arrested on suspicion of punching an 87-year-old woman in the face on a bus.
CCTV footage showed the pensioner being attacked as she travelled on the 166 bus through Croydon in south London.
She suffered a black eye after being punched by one of two teenagers who had boarded the bus without paying, the Metropolitan Police said.
Officers from the Met's Roads and Transport Policing Command arrested the teenager on suspicion of actual bodily harm.
She was taken to a south London police station, where she remains in custody.
Officers have also identified the second female believed to have been involved in the incident, but she has not been arrested at this time, the Met said.
Most of the passengers left the bus after the girls began arguing with the driver, and the elderly victim can be seen on CCTV standing next to the pair before the attack, at around 3.20pm on Friday October 16.
Both girls, described as black and aged between 14 and 18, ran away from the bus in Brighton Road, Coulston.
Another woman passenger came to the aid of the victim, who was taken to St Helier Hospital and discharged later the same day.
Police sergeant Arran Hillier, from the Croydon Safer Transport Team, said: "This was a nasty attack on an elderly woman, who just wanted to get home."