Updated at 3:05 p.m. ET
In a midday attack near the U.K. Parliament building, an assailant killed two civilians and a police officer before being shot to death, British authorities say.
At least 20 people were injured in the attack, which police describe as a terrorist incident. Authorities say they believe there was a single attacker, driving a vehicle and armed with at least one knife.
Witnesses report that a small SUV barreled into pedestrians along the sidewalk of Westminster Bridge, causing multiple "catastrophic injuries" to pedestrians, according to Britain's Press Association. The London Ambulance Service says it treated 10 injured people on the bridge.
Acting Deputy Police Commissioner Mark Rowley said he believes this is where the two civilian deaths occurred. Three police officers, who happened to be returning from a commendation ceremony, were also injured by the vehicle, he says.
Three teenage French students on a school trip to London were also among the wounded, Reuters reports.
A woman was found in the Thames after the attack, alive but injured; how she fell in the river is not clear.
After striking the pedestrians on the bridge, the SUV then crashed into the fence that surrounds Parliament. A man armed with a knife emerged and stabbed a police officer before being shot to death by police.
British law enforcement officers do not typically carry guns, but The Metropolitan Police said on Twitter that armed officers were among the law enforcement personnel "on scene and dealing with the incident."
The officer who was stabbed and died was armed, Rowley said.
The British press reports that a member of Parliament was among the people providing first aid to the police officer who was stabbed. Photographs show Foreign Office Minister Tobias Ellwood, his face bloodied, crouched at the heart of a group of emergency responders.
Ellwood is a former soldier whose brother died in the 2002 terrorist attack in Bali, The Telegraph reports.
The U.K. House of Commons is under lockdown, as NPR's Frank Langfitt in London reports. Here's more from Frank on how the incident unfolded, beginning with the events on Westminster Bridge:
"This is the bridge that goes right up to Big Ben — it crosses the Thames. And it appears that a vehicle went across the bridge [and] went down the sidewalk. And we can see from aerial helicopter shots that in fact there are at least four or five people being treated, lying on sidewalk. Then it came across the bridge [and] crashed into a gate; the front end is smashed up. "And then somehow a man ... got actually inside the gates of the Parliament building and apparently stabbed a police officer."
Authorities have not identified the driver, although the British press have published images of a man they say is the suspected assailant.
House of Commons business was suspended for approximately 15 minutes before Commons Leader David Lidington told members of Parliament about the situation, saying a police officer had been stabbed and the alleged assailant shot by armed police.
During the lockdown, members of the House of Commons nearly filled the chamber's famed green benches — moments earlier, the benches sat empty after a sparsely attended debate over pension plan rules.
MPs later voted to adjourn Parliament for the remainder of the day. But Parliament says it will return to its normal schedule tomorrow.
"For this to happen right in the heart of the city is very shocking," Frank added. It's also surprising that an attacker appears to have gotten into the heavily fortified premises of Parliament.
The nearby Westminster metro station is closed "due to a police investigation," city authorities said, and police have asked members of the public to steer clear of the area.
Additional police have been deployed around the city, Harrington said.
White House press secretary Sean Spicer said in a tweet that President Trump has been briefed on the ongoing situation.
This is a developing story. Some things that get reported by the media will later turn out to be wrong. We will focus on reports from police officials and other authorities, credible news outlets and reporters who are at the scene. We will update as the situation develops.
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