A knifeman reportedly stabbed a passenger on a train at Forest Hill station before chasing people outside shouting "I want to kill a Muslim".
The attacker allegedly shouted: "Who is a Muslim? I want to kill a Muslim."
A 38-year-old man from London has been arrested on suspicion of GBH following the "serious incident", British Transport Police confirmed.
They said they were treating the incident as a "serious hate crime" but said there was "no suggestion of wider terrorist aspect".
An eyewitness told The Daily Mirror: "The man came running out of the station with a knife and everybody started running and screaming.
"He was shouting 'Whoever is Muslim, I will kill you". He was looking for anybody that was Muslim.