Police shut down the area around the Houses of Parliament in central London on Tuesday after a motorist crashed into pedestrians and cyclists and then into a security barrier outside Parliament during the morning commute.
Police later confirmed the incident was a terrorist attack.
The attack heightened tensions in the city, which has seen four vehicle-based terror attacks in less than 18 months.
After the incident, armed police swooped into the area and cordoned off streets surrounding the heart of Britain's government. Police appealed to the public to stay away, and the Westminster Tube station and Westminister Bridge were closed.
"While we are keeping an open mind, the Met's Counter-Terrorism Command is leading the investigation into the Westminster incident," London's Metropolitan Police said on Twitter, before officially labeling the event a terrorist act.
Earlier, police said the male driver of the car had been arrested at the scene of the crash, which occurred at 7:37 a.m.
Eyewitness Ewelina Ochab told The Associated Press that the crash appeared deliberate.
"The car drove at speed into the barriers outside the House of Lords. There was a loud bang from the collision and a bit of smoke," she said.
"The driver did not get out. The guards started screaming to people to move away."
Another witness, Jason Williams, said the car forcefully struck a barrier on a lane used for access to the parliament building. He also said he thought it was deliberate.
"It's a very serious incident," he told reporters. "There was smoke coming from the vehicle."
Images shot by a Euronews journalist showed police pointing their guns at a vehicle. Footage on social media showed a handcuffed man being led away by heavily armed police. Other footage showed a cyclist lying on the street.
"I saw the cyclists, injured cyclists. I've seen people, about 10, on the road, lying down, but I haven't seen any fatalities," Williams said.
London's Ambulance Service said it had treated two people for injuries at the scene and they had been taken to hospital. Their injuries were not believed to be serious.
Parliament is in the midst of its summer recess and most lawmakers were not using the building. British Prime Minister Theresa May is on holiday abroad.
In March 2017, Khalid Masood, 52, killed four people on nearby Westminster Bridge and stabbed an unarmed police officer to death in the grounds of Parliament before being shot dead. It was the first of five attacks on Britain last year that police blamed on terrorism.
Britain is on its second highest threat level of "severe," meaning a terrorist attack is considered highly likely.
Last week, a Muslim convert admitted plotting to kill more than 100 people by driving a truck into pedestrians on London's Oxford Street, the capital's major shopping thoroughfare.
In October last year, 11 people were injured when a car collided with pedestrians near London's Natural History Museum, raising fears of an attack, but police later said the incident was a traffic accident.
