A man with a knife stabbed another in an east London subway 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. I would urge the public to remain calm, but alert and vigilant," said Richard Walton, who leads the London police's counter-terrorism command.
Police were called in response to reports of a number of people stabbed at the Leytonstone 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.
Several British newspapers quoted eyewitnesses to the incident saying the man shouted "this is for Syria," but police declined to comment on any reports.
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 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 legislators approved the bombing of Islamic State targets in Syria on Wednesday. Britain's air force has since carried out two bombing raids.