Four people were killed, including the assailant, and about 40 others were injured in what Prime Minister Theresa May condemned as a "sick and depraved" attack outside the UK parliament building.
At least four people were killed and 40 others were injured after an assailant ploughed his car into pedestrians on London's Westminster Bridge then leapt out and stabbed a policeman outside the parliament building on Wednesday, officials said.
The assailant was shot dead by police as the parliament building was put on lockdown.
London Police said that officers were called to an incident on Westminster Bridge near to where witnesses said a vehicle struck several people; photos of the crime scene showed a car that ploughed into railings.
The injured included three French schoolchildren, authorities in France confirmed.
London Metropolitan Police said in a statement that they were treating the situation as a "terrorist incident" until they know otherwise.
Prime Minister Theresa May, who was at parliament when the incident began, condemned the attack as "sick and depraved."
"The location of this attack was no accident," she said in a statement outside her Downing Street office late in the evening.
The former Polish foreign minister Radoslaw Sikorski, who was on the bridge, posted a video on Twitter that showed people lying injured.
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.
A car on Westminster Bridge has just mowed down at least 5 people. pic.twitter.com/tdCR9I0NgJ— Radosaw Sikorski (@sikorskiradek) March 22, 2017
The threat level for international terrorism in the UK was listed at severe. The city was also on alert for the Thursday funeral of Martin McGuinness, former IRA commander.
There was no immediate claim of responsibility for the attack, which happened on the first anniversary of the Daesh suicide bombings in the Brussels airport and subway that killed 32 people.
The scene unfolded within clear sight of the London Eye, a large ferris wheel with viewing pods with views of the capital. Footage showed the pods full as viewers watched police and medical crews on the bridge.
TRT World's Sarah Morice brings more from near Westminster Bridge in London.
World leaders react
Turkey's President Recep Tayyip Erdogan expressed the country's "condolences" and said, "On behalf of my nation and myself, I offer my sincere condolences to the friendly people of the United Kingdom, close relatives of the people who lost their lives in this attack. I wish immediate emergency healing for injured people."
German Chancellor Angela Merkel said Germany stood by Britain "firmly and resolutely."
In his message to the UK leadership French President Francois Hollande said, "France, which has been hit so hard in recent times, knows what the British people are suffering today."
US Secretary of State Rex Tillerson expressed his "condolences to the victims and their families" in a statement, addi, "The American people send their thoughts and prayers to the people of the United Kingdom."
Our thoughts are with the victims of todays attack in London & their families. Canadians remain united with the people of the UK.— Justin Trudeau (@JustinTrudeau) March 22, 2017
European Commission President Jean-Claude Juncker said that the attack left him "highly emotional."
Maria Zakharova, spokeswoman for the Russian foreign ministry, expressed the country's "condolences" and said on Russian television, "We do not and will never support terrorism."
"We consider it an evil to which it is necessary to fight collectively. In this moment, as always, our hearts are with Britons and we share their pain."
Dutch Prime Minister Mark Rutte told Dutch television he was shocked by the "horrific" news from London and said, "The city is in our hearts. We are following the situation closely, and are in contact with the British authorities."
My thoughts are with the victims of the Westminster attack. Europe stands firm with the UK against terror and ready to help.— Donald Tusk (@eucopresident) March 22, 2017
Italian Prime Minister Paolo Gentiloni expressed his "condolences" and said, "Italy and the United Kingdom remain side-by-side in our condemnation and firm response to all forms of terrorism."
"Our condolences are with those who mourn and all who are affected in London," Belgian Prime Minister Charles Michel tweeted. "Belgium stands with UK in fight against terror."
"We express our solidarity with the British people on today's indiscriminate attack," Greek Prime Minister Alexis Tsipras tweeted.
Spanish Prime Minister Mariano Rajoy tweeted, "Spain stands with the British people. I condemn the attack in the vicinity of Westminster, London. Solidarity with the victims."
Norwegian Prime Minister Erna Solberg tweeted, "Terrible to see an attack in the heart of the British democracy. My thoughts with the victims."
Pakistan condemns terrorist incidents in London today. Our thoughts & prayers are with the families of the victims & those injured.— M. Nafees Zakaria (@ForeignOfficePk) March 22, 2017
Meanwhile French authorities said that lights on the Eiffel Tower will be switched off at midnight (2300 GMT) on Wednesday in solidarity with those killed and injured in the attack.
The decision was taken by Paris Mayor Anne Hidalgo who also expressed her "solidarity" in a message to her London counterpart Sadiq Khan, adding, "Paris and London have very close links which have become stronger in the last few decades."