A driver hit pedestrians and cyclists before crashing into barriers outside the Houses of Parliament in London on Tuesday morning.
The episode appeared to be a "deliberate act" and is being treated as a terrorist attack.
A male suspect in his 20s was arrested.
He is in detention but "not currently cooperating," according to the police.
Three people are injured, but they are not believed to be seriously hurt.
US President Donald Trump condemned the attack and called for "toughness and strength."
Three people were injured Tuesday morning by a driver who eventually crashed into barriers outside the Houses of Parliament in London.
The silver Ford Fiesta hit several cyclists and pedestrians before hitting the barriers at 7:37 a.m. local time.
The police arrested the vehicle's driver, identified as a man in his late 20s, at the scene on suspicion of terrorist offenses.
Neil Basu, the head of the UK's counterterrorism unit, told reporters that the episode appeared to be a "deliberate act" and was therefore being treated as terrorism. He added that the suspect, who is detained, "is not currently cooperating."
The car remains on the scene and is being searched. The police have not found any weapons.
Three people were treated at the scene, with two of them taken to a hospital. The London Ambulance Service said the injuries were not believed to be serious.
The British government is set to hold an emergency meeting for its Cabinet Office Briefing Rooms committee, also known as Cobra, to discuss the attack at 2 p.m. on Tuesday.
US President Donald Trump characterized the incident as an attack, tweeting: "Another terrorist attack in London...These animals are crazy and must be dealt with through toughness and strength!"
Sky News broadcast footage appeared to show a man, who the network circled in red, being detained after the crash.
A witness, named James, told LBC News:
"I was coming up to Westminster on a Santander bike. On the right-hand side of the road as you come up to Parliament, there was a female cyclist on the floor injured with others surrounding that.
"An ambulance turned up and tried to attend to the person. I was going to continue towards Parliament, but was told to get off my bike.
"Considering recent events, I wanted to get out of the area. I returned the way I had come and then cycled around the back of Parliament.
"There was smoke coming out of the car and there were suspicions there was a device, but that was unconfirmed, I was just told that by another cyclist.
"They also said the car went across the road."
Videos tweeted by Sky News and the Euronews/NBC UK journalist Vincent McAviney showed heavily armed police officers at the scene as a man was led from the car in handcuffs.
The police briefly closed Westminster Station and cordoned off several streets around the site of the attack.
In 2017, a 52-year-old Briton named Khalid Masood rammed his car into pedestrians on the pavement along the south side of the Westminster Bridge, injuring more than 50 people and killing five.
Masood crashed his car into the fence outside New Palace Yard and fatally stabbed an unarmed police officer before being shot dead.
The police stepped up security measures around Westminster after Masood's attack and another terror attack on London Bridge last year. Extra barriers were put in place, and armed officers were given permission to shoot at moving vehicles found to be involved in attacks, ITV News reported.
The Met Police also rolled out new equipment, named the "Talon," that would place steel spikes on roads to prevent the vehicle-ramming attacks. It's not clear whether the Talon was used Tuesday.