A 14-year-old boy has been charged with two counts of inciting a child to engage in sexual activity and two counts of kidnap.
Two teenage boys told officers they were forced to fight each other and later to perform a sex act as the suspect watched them.
The two victims were travelling on the upper deck of a route 133 bus in the Elephant and Castle area on 11 February when the suspect approached them. Metropolitan Police detectives from the Sexual Offences Exploitation and Child Abuse Command said the suspect threatened the boys and told them he was armed with a knife.
He then forced them off the bus and they were led to a park in Newington Butts.
There, they were made to fight each other and steal from a nearby shop.
The suspect is then alleged to have taken them to the stairwell of a block of flats, where he forced one of the boys to perform a sex act on the other.
On 19 March, police released CCTV footage of a suspect.
The youngster, from south London, will appear in custody at Camberwell Youth Court on 21 March.
Police have praised the bravery of the boys in coming forward after their ordeal.
Trainee Detective Constable Amy Beautridge said: "Despite their young age, the victims have been incredibly brave in coming forward and providing information to the police and I would praise them for this."