A man who allegedly stabbed a train passenger was heard shouting "death to Muslims" and "go back to Syria" remains in police custody.
Police were called at 1.18pm on Monday to the incident, which was believed to have happened on a moving train between Honor Oak Park and Forest Hill station in south-east London.
A man in his 40s who was injured in the attack was taken to hospital where he remains in a serious, but not life-threatening condition, British Transport Police (BTP) said.
A 38-year-old man was arrested on suspicion of causing grievous bodily harm and remained in custody on Monday night.
BTP said they were treating the incident as a "serious hate crime" and said there was "no suggestion of wider terrorist aspect".
Superintendent Jenny Gilmer said: "This is an isolated incident and at this time is being treated as hate crime. We have no reasons to believe that it has any links to terrorism.
"Investigations are ongoing and officers are working to establish the circumstances around today's events."
Miguel Oliveira, 36, said he came "face to face" with the attacker at Forest Hill station, who was "shouting uncontrollably" after he allegedly stabbed the victim.
Mr Oliveira, a music composer, told the Press Association how he was met with a "wave of people" when he arrived at the station following the incident.
Passengers were shouting, "He's got a knife, he's got a knife" and "He wants to kill Muslims", he said.
Mr Oliveira, from Forest Hill, left the station but returned a few minutes later amid confusion over the whereabouts of the attacker.
He said: "I doubled back and I went back to the station and actually came face-to-face with him.
"He had just come out - not the way they were saying.
"He was walking towards me and he was shouting stuff like, 'Death to Muslims' and 'Go back to Syria'."
Eyewitness Richard Etienne said the train had just left Honor Oak Park when he heard someone shout, "I want to kill me a Muslim".
The man, who was walking with "serious intent", then made his way from the back of the train into Mr Etienne's carriage, he told radio station LBC.
He said: "As he walks towards me, he looks at me right in my eye and then stopped and looks at the woman right opposite me wearing a hijab.
"And he now - at this point - turned and faces her so I just have his back to me and I cannot see the woman.
"I can see that he's brandished a knife towards her. It looked about five inches in length."
The attacker then shouted at her, "I don't want me a Muslim woman, I want me a Muslim man", Mr Etienne said.
The man then moved up the train into the first carriage, continuing to shout.
"The next thing I hear is a scream and I can see about three of four guys jumping towards the scream. So my assumption is they tried to stop the guy and hold him down," he said.
After the train pulled in at the station, the attacker walked "calmly" away but was "still shouting", Mr Etienne added.
Shellby Curry, 24, from Forest Hill, 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*****g 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.
Olusegun Adegbiyi, controller of Fare Cars, a taxi service at Forest Hill station, said he saw people running outside after the attack.
"As they were running this man, on his own, came running after them and stood just short of where they went," he told the Press Association.
"He kept saying to them, 'You don't have to be scared of Muslims, you don't have to be scared of Muslims'."
Police are appealing for anyone who saw the incident to come forward.
Detective Chief Inspector Paul Langley, BTP senior investigating officer, said: "This was a serious assault on a man in broad daylight and we are aware that other passengers may have seen what happened or been threatened themselves.
"We have spoken with many witnesses from the train but if you saw what happened and haven't yet contacted police, please do so as soon as you can."