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.
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.
"Everybody was running and screaming. He was walking up and down the street for about 15 minutes.
"The looks on some peoples faces, they looked terrified and very scared."
Other eyewitnesses said the man was shouting "death to Muslims" and "go back to Syria".
Miguel Oliveira told the Press Association he was met with a "wave of people" when he arrived at the station.
"The first few were shouting 'he's got a knife, he's got a knife' and then all of a sudden the next ones said 'Oh he's been stabbed call the police'," he said.
He added that some members of the crowd were shouting: "He wants to kill Muslims".
Shellby Curry was with her one-year old child when she saw a man "waving what looked to be a knife in his hand" near the station and screaming words like "Muslims f***ing hate them, kill them all".
She described the attacker as a black man with short hair, who she thought appeared to be in his late 30s or early 40s.
The man also threatened a woman wearing a hijab before the stabbing, LBC News reports.
One man, who is in his forties, was treated by ambulance staff at the scene and then taken to hospital, British Transport Police confirmed.
A BTP 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.
"We are still investigating the circumstances of this incident but at this time, we are not treating it as terrorism-related."
