A 15-year-old boy has been arrested on suspicion of attempted murder after a boy of the same age was stabbed in south London, Scotland Yard said.
Detectives are appealing for information and witnesses following the incident in Sutton.
Police were called to St Helier Avenue at 2.35pm yesterday after a stabbing on a Route 151 bus.
Officers, London Ambulance Service and London's Air Ambulance were at the scene.
The victim was taken to hospital where his condition is serious but stable.
The Metropolitan Police said a 15-year-old boy was arrested on suspicion of attempted murder. He was taken to a south London police station where he remains in custody.