(Adds statement from police counter terrorism head, background)
LONDON, Dec 5 (Reuters) - A man with a knife stabbed another in an east London metro station on Saturday evening, reportedly screaming "this is for Syria", before police used a stun gun on the attacker and detained him.
"We are treating this as a terrorist incident," Richard Walton, who leads the London police's Counter Terrorism Command, said in a statement.
Police were called to reports of a number of people stabbed at the Leytonstone Underground station in east London and a man threatening other people with a knife. One man was seriously injured and two sustained minor injuries, police said.
London's Evening Standard newspaper said the attacker had screamed "this is for Syria". A police spokesman declined to comment on the report about Syria.
Britain is on its second-highest alert level of "severe," meaning a militant attack is considered highly likely. This is mainly due to the threat the authorities say is posed by Islamic State militants in Syria and Iraq and their encouragement of supporters to carry out attacks in their homelands.
British lawmakers approved the bombing of Islamic State targets in Syria on Wednesday. Britain's air force has since carried out two bombing raids.
Last month militant Islamist attacks in Paris killed 130 people.
Britain suffered by far its worst militant Islamist attack in July, 2005, when 52 people were killed by suicide bombs on underground trains and a bus.
In May 2013, a British army soldier Lee Rigby was attacked and killed by two Muslim converts on a busy street in southeast London, a murder that provoked disgust and fears of an anti-Muslim backlash. (Reporting by James Davey and Guy Faulconbridge; Editing by Chris Reese and Chizu Nomiyama)