LONDON — A man armed with a knife and wearing a fake explosive device strapped to his body was shot and killed by the police in South London on Sunday after he was suspected of stabbing people on Sunday.

The authorities described the attack as “terrorist related.”

The assault in London came more than three months after another terrorism spree left two people dead near the London Bridge, and added to the litany of attacks Britain has endured in recent years, testing the nerves and fortitude of its residents.

According to Lucy D’Orsi, the deputy assistant commissioner in specialist operations, the suspect was Sudesh Mamoor Faraz Amman, 20.

Local news reports said Mr. Amman had been under surveillance by the police when two pedestrians were stabbed on Sunday in Streatham, about five miles south of Central London. One of them, a man, was hospitalized with “life-threatening” injuries, according to the police.