March 22 (UPI) -- What's being treated by police as a "terrorist incident" rattled central London on Wednesday, leaving four people dead at the central nerve center of Britain's legislative government.
The incident began around 2:40 p.m. in London's Westminster district when the attacker drove an SUV into pedestrians on Westminster Bridge.
Moments later, the 4x4 crashed into the front gates at the Palace of Westminster, where the assailant jumped out and fatally stabbed a police officer, authorities said. The suspect was shot dead on the spot by nearby police before he could enter the building. His identity was not immediately revealed.
"Two plain-clothes guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," witness and journalist Quentin Letts said.
Both parliamentary houses -- the House of Commons and House of Lords -- meet at the Palace of Westminster. It is commonly called the Houses of Parliament.
"Police officers were running with machine guns, and there was a man down right next to the car," Bradford Buck, a visitor from Connecticut, told BBC News.
"We saw loads of people ducking down," witness George Nuth told NBC News. "Police or security were telling people to run. A policeman said to us, 'Stop dawdling if you don't want to get shot.'"
"We are treating this as a terrorist incident until we know otherwise," London's Metropolitan Police said in a statement.
Within minutes of the attack, eight armed undercover police officers rushed British Prime Minister Theresa May from Parliament to a waiting vehicle.
Investigators believe the suspect acted alone in the assault, which occurred on the one-year anniversary of an attack in Belgium that killed 32 people.
"The terrorist chose to strike at the heart of our national capital," May said outside her Downing Street residence, calling the attacker "sick and depraved."
"The location of this attack was no accident."
Authorities placed the parliamentary building and its surrounding area, including Westminster Bridge, on lockdown in the moments after the attack. Those areas were later evacuated.
Metropolitan Police said it deployed extra officers around London as a precaution.
"This is a day that we planned for but hoped would never happen. Sadly it has now become a reality," Asst. Police Commissioner Mark Rowley said.
"Today, London suffered a horrific attack," Mayor Sadiq Khan said in a video statement. "My heart goes out to those who have lost loved ones and to everyone who has been affected.
"I want to reassure all Londoners and all our visitors not to be alarmed. Our city remains one of the safest in the world. ... And we stand together in the face of those who seek to harm us and destroy our way of life."
"He pledged the full cooperation and support of the United States government in responding to the attack and bringing those responsible to justice," the statement said.
French Prime Minister Bernard Cazeneuve said some high school students visiting London from France were injured in the attack.
Scotland Yard's counter-terrorism command is leading the investigation, officials said, adding that British Parliament will meet Thursday as scheduled.
A car on Westminster Bridge has just mowed down at least 5 people. pic.twitter.com/tdCR9I0NgJ— Radosław Sikorski (@sikorskiradek) March 22, 2017