A pool of blood near the ticket barriers at the Leytonstone Underground station, about six miles (10 km) east of central London, could be seen in footage posted on Twitter that also showed the suspect confronting police on Saturday evening.
Police said the man, believed to be aged 29, had also threatened other bystanders. One man, thought to be 56 years old, suffered serious but not life-threatening injuries and was in a stable condition at a London hospital. A second victim suffered minor injuries.
"I am treating this as a terrorist incident," Richard Walton, who leads the Counter Terrorism Command at London`s Metropolitan Police, said in a statement. The man was arrested after police used a Taser to subdue him.
Detectives were searching a residential address in east London and additional officers were being deployed to the rail transport network, police said.
An eyewitness quoted by the Guardian and other British newspapers said the attacker appeared to claim that he was retaliating for Western air strikes on Islamist militants in Syria, shouting: "This is what happens when you f*** with mother Syria, all of your blood will be spilled!"
Police declined to comment on those reports and it was not immediately possible to verify them independently.
British war planes joined the strikes for the first time on Thursday, a few hours after Prime Minister David Cameron won parliamentary approval to bomb the Islamic State militant group in Syria after it claimed responsibility for attacks on Paris last month that killed 130 people.
In an interview with the Sunday Times, Syrian President Bashar al-Assad mocked Cameron`s strategy, saying it would make the situation worse, not better.
THREAT TO BRITAIN
Britain is on its second-highest security alert level of "severe", meaning a militant attack is considered highly likely, though not imminent, mainly because of the threat posed by Islamic State militants in Syria and Iraq.
Secretary of State for Work and Pensions Iain Duncan Smith said that, whatever the circumstances, Britons must not let the Leytonstone incident affect their behaviour.
"We cannot let these sort of people, terrorists et cetera, actually dominate our space," he told the BBC. "The way we defeat them at the end of the day is with our values, our freedom of expression, our freedom of belief ... our ability to take our children, our families out at Christmas. None of that must be curtailed."
Nevertheless, the attack will draw parallels with the May 2013 murder of British army soldier Lee Rigby, who was hacked to death in east London by two Muslim converts.
Cameron has said air strikes will not increase the chance of an attack on Britain, since militants already view it as a top target, with seven plots foiled over the past year.
Britain`s worst Islamist militant attack was in July 2005, when 52 people were killed by suicide bombs on underground trains and a bus.
Islamic State said on Saturday that the married couple who killed 14 people in a mass shooting in California were its followers.
Knife attacker slashes man in London `terrorist incident`
07 December 2015
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A knife attacker slashed a man at an east London metro station, reportedly screaming "This is for Syria!", in what police described as a terrorist incident, prompting a senior minister to urge Britons on Sunday not to be intimidated.
