Police said they had arrested a 38-year-old man following the attack on a train which left the victim, who is in his forties, in serious condition in hospital. An eyewitness, 36-year-old Miguel Oliveira, told the Press Association news agency that he came face-to-face with the attacker, who was "shouting uncontrollably".
"He was walking towards me and he was shouting stuff like 'Death to Muslims' and 'Go back to Syria'," he said. British Transport Police said it had received reports that a man had been assaulted on a train at Forest Hill station in southeast London at 1:18pm (1318 GMT).
"The man, who is in his forties, was taken to hospital with injuries consistent with having been stabbed. He remains in hospital in a serious, but not life threatening, condition," it said in a statement. "A 38-year-old man from London was arrested at the scene on suspicion of assault occasioning grievous bodily harm. He remains in police custody. "The circumstances are being investigated but the incident is not being treated as terrorism-related at this time."