A man has reportedly been stabbed on a train at Forest Hill station before the attacker chased people outside with a knife.
The attacker has been arrested on suspicion of GBH, British Transport Police confirmed.
One man was treated by ambulance staff at the scene and then taken to hospital.
Eyewitnesses said officers had been called to arrest a suspect after at least one person was attacked.
One witness told The Evening Standard: “I spoke to my daughter’s friend who was on the train and she said a man with a knife was going around and stabbing people
“She said everyone literally ran off the train at Forest Hill where he got off as well.
“He had a wild look in his eye."
A British Transport Police spokeswoman said: "We are currently at Forest Hill railway station, after being called to a serious assault at 1.18pm.
"One person has been arrested following the incident and is currently in police custody.
"One man was treated by ambulance staff at the scene and is currently en route to hospital."
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