A knifeman launched a terrorist attack at an east London tube station seriously injuring one man and wounding another on the evening of Saturday, 5 December. He reportedly shouted "this is for Syria" during the attacks.
"We are treating this as a terrorist incident," said the leader of the Metropolitan Police's Counter Terrorism Command. "I would urge the public to remain calm, but alert and vigilant."
A spokeswoman for 10 Downing Street said they were monitoring the situation. Boris Johnson, the mayor of London had been briefed on the incident.
Officers were called to Leytonstone station at 7.06pm with reports of a stabbing, the Met said in a statement.
Dramatic mobile phone footage shot by a number of commuters showed him appearing to attack a man as police arrived on the scene. One Twitter user posted a series of graphic videos showing the aftermath of the attack.
One witness posted a video of the attack on YouTube video. It shows a man, wearing mustard trousers and a grey sweatshirt, who appears to have a small blade in his hand. As police and passengers walk towards him he charges at them before the police attempt to taser him a number of times, eventually succeeding.
Panicked commuters, can be heard saying: "I just want to get out of here," before fear turns to anger and a man shouts: "You ain't no Muslim bruv" as police handcuff him. The phrase began trending on Twitter shortly afterwards as people praised the sentiment.
Other footage captured a huge pool of blood on the floor.
A witness called Salim who works at a shop in station told London's LBC Radio that he initially thought the screaming and shouting was from drunk people until he heard the victim shouting "somebody help, somebody help".
"The tall man was punching him so hard and when they fall on the floor he started kicking him and then I called the police," he said. "When he started stabbing or cutting something on his body, everybody ran away and only three of us were here in the concourse. It was scary and it was empty, nobody was there."
Another witness, Michael Garcia, told the BBC that he saw people running outside the station before he came across "a guy, an adult, lying on the floor with a guy standing next to him brandishing a knife of about three inches… maybe a hobby knife".
The blade was thin but fairly long, the financial analyst from Leytonstone said, adding that the knifeman was "screaming 'go on, then, run' to everyone else".
A man believed to be aged 29, was arrested at 7.14pm after officers discharged a taser at him. He was taken to an east London police station where he remains in custody, police said.
"Officers gave first aid to a 56-year-old man who sustained serious knife injuries," they said, adding that he was taken to an east London hospital where he remains in a stable condition. "His injuries are not believed to be life-threatening," they said.
Detectives also began searching a residential address in east London, they added.
A second man sustained a minor injury during the incident although he did not require medical assistance and a woman was threatened by the suspect but was not injured, they added.
If police decide he should face terrorism charges, it would be the first terrorist attack on British soil since the murder of fusilier Lee Rigby on 22 May, 2013.
Labour leader Jeremy Corbyn and Tim Farron, the leader of the Liberal Democrats took to Twitter to condemn the attack.
Corbyn call it "absolutely shocking" while Farron said it "was an act of pure evil".