LONDON — Attacks on civilians in two locations near the Thames River on Saturday night sent armed London police officers to each site.
Theresa May, the British prime minister, said the authorities were treating the attacks “as a potential act of terrorism,” and the London police declared the attacks “terrorist incidents” a short time later.
Authorities closed London Bridge after a white van mounted the sidewalk and mowed down pedestrians a little after 10 p.m., according to multiple witnesses on the bridge.
The police said there were casualties there, with more than one person dead, and witnesses reported seeing a bearded man with a large knife.
Soon after, there were reports of knife attacks at nearby Borough Market, a popular spot for pubs and restaurants on the southern side of the Thames. The police said that “armed officers responded and shots were fired.”
Police advised people in each area to run to a place of safety, hide, silence their telephones and call the police when they could.
A third incident, at Vauxhall, which was also a stabbing, was not connected to the London Bridge or Borough Market attacks, the police said.
Downing Street announced that May would chair a meeting of the government emergency response committee, known as Cobra, today.
Holly Jones, a BBC reporter who was on the bridge at the time of the incident, said the van was driven by a man and was “probably traveling at about 50 miles an hour.”
“He swerved right round me and then hit about five or six people,” Jones said. “He hit about two people in front of me and then three behind.”
Police and ambulance workers responded rapidly, evacuating pedestrians and shutting down the London Bridge subway station. Jones later said that she saw a man being arrested by the police.
The police also sent armed officers to Borough Market, where witnesses reported stabbings.
A witness, who identified himself only as Andrew, said he was in the area at a bar, heard “a massive bang” and saw a van hitting a railing.
Mark Roberts, who said he was on London Bridge at the time, told CNN that the van was traveling south across the bridge toward the London Bridge station.
The van was speeding and swerving, Roberts said.
Roberts said that the van hit several people on the bridge and that about 10 minutes later he heard what sounded like 10 to 20 gunshots.