(CN) - Four people have died and 40 people suffered injuries Wednesday, after a lone attacker struck them with a vehicle on Westminster Bridge, near the British Parliament building, and then commenced to stab onlookers and at least one police officer.
The British Parliament was on lockdown for several hours after the incident, but people have been seen leaving the building in the past half hour.
The head of counterterrorism at London's Metropolitan Police, said the four people who have died include an attacker and a police officer.
Mark Rowley said some 20 people have been wounded and a search is underway to make certain no other attackers are in the area — though police believe there was only one attacker.
Rowley said the dead policeman was one of the armed officers who guard Parliament. The other victims were on Westminster Bridge.
Rowley says "We are satisfied at this stage that it looks like there was only one attacker. But it would be foolish to be overconfident early on."
The leader of Britain's House of Commons said a man has been shot by police at Parliament. David Liddington also said there were "reports of further violent incidents in the vicinity." A session of Parliament was suspended after the incident.
British lawmaker Grant Shapps said on Twitter that he was walking through the cloisters of the House of Commons to vote when he heard four gunshots. Police told lawmakers to get down on the ground and crawl to cover.
"Police response instant. Heard commotion, looked round. Police weapons drawn, 4 shots, police ordered us to hit ground & get back, get back," he said.
Witness Rick Longley told the Press Association that he saw a man stab a policeman outside Britain's Parliament.
"We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out," he said.
"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.
"A guy came past my right shoulder with a big knife and just started plunging it into the policeman.
"I have never seen anything like that. I just can't believe what I just saw."
The former Polish foreign minister Radoslaw Sikorski posted a video on Twitter that seems to show people lying injured in the road on Westminster Bridge.
Sikorski, a senior fellow at the Harvard Centre for European Studies, says he saw at least five people lying on the ground after being "mown down" by a car.
Sikorski told the BBC he "heard what I thought what I thought was just a collision and then I looked through the window of the taxi and someone down, obviously in great distress.
"Then I saw a second person down, and I started filming, then I saw three more people down, one of them bleeding profusely."
The Scottish Parliament in Edinburgh suspended its debate on a second independence referendum after the incident outside the British Parliament in London.
Scottish lawmakers had been planning to vote after two days of debate on First Minister Nicola Sturgeon's call for another referendum on leaving the United Kingdom.
The debate was suspended after some members said it should be halted out of respect after a policeman was stabbed and his attacker shot in London.
Sturgeon tweeted that her thoughts were with everyone in Westminster "caught up in this dreadful incident."
The White House said President Donald Trump spoke with British Prime Minister Theresa May by phone to offer his condolences and praise her government "for the effective response of security forces and first responders."
Trump went on to pledge the full cooperation and support of the United States government in responding to the attack and bringing those responsible to justice, the White House said.
Trump's spokesman, Sean Spicer, says the U.S. will continue to monitor the situation and update the president. - Developing story.
The Associated Press contributed to this report.
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