A knifeman who reportedly stabbed a passenger at Forest Hill train station shouted "death to Muslims" and "go back to Syria" during the attack, eyewitnesses said.
The man who was stabbed has been taken to hospital but his condition is currently unknown.
Witness Miguel Oliveira, 36, said he came "face-to-face" with the attacker, who was "shouting uncontrollably".
He said he was met with a wave of people as he arrived at the station in south east London.
"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 added that some members of the crowd were shouting: "He wants to kill Muslims".
Another witness, Shellby Curry, who lives locally to the station, was with her one-year-old child when she saw the attacker.
She said he was "waving what looked to be a knife in his hand" near the station and screaming words like "Muslims f****** hate them, kill them all".
"I just grabbed my baby and ran across the road. Looking back [I saw] him marching up and down waving his arm about," she said.
She described the attacker as a black man with short hair, who she thought appeared to be in his late 30s or early 40s.
Several people phoned the police, who arrived around 10 minutes later and restrained him, Miss Curry said.
Police said a suspect has been arrested on suspicion of GBH after the stabbing which happened just after 1.15pm.
A British Transport Police spokesman said: "We are investigating the circumstances of the Forest Hill incident, but at this time we are not treating it as terrorism-related."
Other passengers posted what they had seen on Twitter, with many claiming the man had attacked a Muslim passenger on a train with no provocation.