Four people died and 20 others were injured in the terror incident near the British Parliament in London on Wednesday, a security official said.
The head of counterterrorism at London's Metropolitan Police, Mark Rowley, said four people have died in the incident, including an attacker and a police officer.
Rowley said some 20 people have been wounded in the attacks that involved mowing by a vehicle. The Parliament was locked down a search is underway to make certain no other attackers are in the area — though police believe there was only one attacker.
A vehicle mowed down pedestrians on London's Westminster Bridge, killing at least one woman and leaving others with injuries described as catastrophic. Around the same time, a knife-wielding attacker stabbed a police officer and was shot on the grounds outside Britain's Parliament, sending the compound into lockdown.
The attack came exactly a year after the Brussels suicide bombing. Wednesday, March 22, was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people.
The threat level for international terrorism in the U.K. was already listed at severe when the attack took place.
The latest event echoed recent vehicle attacks in Berlin and Nice, France.
There was no immediate claim of responsibility and it was not clear if there was more than one attacker.
The incident in London unfolded within sight of some of the city's most famous tourist sites, including the London Eye, a large Ferris wheel with pods that overlook the capital. It stopped rotating and footage showed the pods full as viewers watched police and medical crews on the bridge, which has at its north end Big Ben and Parliament, two iconic symbols.
"The whole length of the bridge there were people on the ground," Richard Tice, a witness, told Sky News. The London Ambulance Service said it had treated at least 10 people on the bridge, and British port officials said a woman was pulled from the River Thames, injured but alive.
At Parliament, a body was seen lying in the yard. It wasn't clear if it was the attacker.
British security has thwarted some 13 terror plots over the past four years, but the UK has largely been spared major international terror attacks such as the ones seen in Belgium and France.
Last year, a far-right supporter shot and killed British lawmaker Jo Cox, who had campaigned for the U.K. to remain in the European Union. Prior to that, an attacker stabbed three people at a train station in east London in response to the Royal Air Force's bombing of the Islamic State group in Syria.
The most gruesome recent attack occurred in 2013 when two Muslim converts of Nigerian descent attacked Lee Rigby, a British Army soldier who was walking down the street. The men ran Rigby down with their vehicle and then used a cleaver to hack him to death as bystanders watched in horror.
The worst peace time attack on Britain this century was on July 7, 2005, when four Al-Qaida-inspired bombers blew themselves up on three subway trains and a bus in London, killing 52. Three of the bombers were British-born, all of Pakistani descent; the other emigrated from Jamaica.
(With AP inputs)