LONDON — A knife attack in a London subway station on Saturday was being treated as a “terrorist incident,” the police said after they arrested a 29-year-old man suspected of the assault.

A 56-year-old victim sustained serious, but nonlife-threatening injuries, and two others were wounded less seriously at the Leytonstone station in East London. The suspect was detained after officers used aTaser on him.

Police refused to comment on why the attack was being investigated as suspected terrorism, or on social-media reports that the assailant had said that he was acting in response to Western involvement in Syria.

“I would urge the public to remain calm, but alert and vigilant,” Cmdr. Richard Walton, who leads the Counter Terrorism Command of London’s Metropolitan Police, said in a statement. “The threat from terrorism remains at severe, which means that a terrorist attack is highly likely.”

Videos that appeared to show the aftermath of the attack were posted online.

Jeremy Corbyn, the leader of the opposition Labour Party, described the stabbing as “absolutely shocking” in a statement on Twitter.