Leaders of countries affected by recent terror attacks have voiced solidarity with the UK after the deadly attack near the Houses of Parliament.
A lone attacker was shot dead after he used a car to run down pedestrians, killing three, and stabbed a police officer to death outside Parliament.
Leaders of France and Germany, which suffered deadly vehicle attacks last year, offered the UK their support.
The US president offered condolences and praised UK security forces.
Among the 40 people injured by the car on Westminster Bridge are three French schoolchildren and two Romanians.
South Korea also said five of its nationals were hurt in the chaos in the wake of the attack.
In Paris, the lights of the Eiffel Tower went out from midnight (23:00 GMT) in a tribute to the victims.
President Francois Hollande expressed his “solidarity” with the British people, saying “terrorism concerns us all and France knows how the British people are suffering today”.
In July last year, a man drove a lorry into pedestrians in the southern French city of Nice, killing 84 people. The attack was claimed by the Islamic State (IS) group.
German Chancellor Angela Merkel, whose country saw a lorry attack in December that killed 12 people in Berlin and was also claimed by IS, said her thoughts were “with our British friends and all of the people of London”.
“I want to say for Germany and its citizens: We stand firmly and resolutely by Great Britain’s side in the fight against all forms of terrorism,” she added.
US President Donald Trump spoke by phone to British Prime Minister Theresa May to offer his condolences and to praise the effective response of UK security services.
Mr Trump pledged the “full co-operation and support” of the US government in bringing those responsible for the attack to justice, the White House said in a statement.
Belgium’s prime minister sent a message of support as his country marked the first anniversary of the suicide bomb attacks on the Brussels airport and underground system, which killed 32 people.
“Our condolences are with those who mourn and all who are affected in London,” Charles Michel tweeted. “Belgium stands with UK in fight against terror.”
European Commission President Jean-Claude Juncker said in a statement that the people of Brussels and Belgium had “suffered a similar pain and felt the support of your sympathy and solidarity”.
“At this emotional time, we at the European Commission can only send that sympathy back twofold.”
In other reaction:
- Canadian Prime Minister Justin Trudeau said in a statement that it was an “attack on democracies around the world” and Canadians stood “united with the British people in the fight against terrorism”
- Australian Prime Minister Malcolm Turnbull said it was an “attack on parliaments, freedom and democracy everywhere” and offered his support and solidarity to the British government
- Chinese Premier Li Keqiang said he and Mr Turnbull had discussed the attack as they met in Canberra. “Together, we send our condolences to the prime minister of the UK and together we condemn terrorism and we stand against all forms of terrorism.”
- Dutch Prime Minister Mark Rutte said: “Horrible images from London. The very heart of the city has been struck. Our thoughts are with the British people.”
- Russian foreign ministry spokeswoman Maria Zakharova expressed sympathy for the injured and offered condolences to the relatives of those who had died, adding: “We don’t split terrorism into categories; we consider it as absolute evil. At this moment, as always, our hearts are together with the British people.”
- Spanish Prime Minister Mariano Rajoy tweeted “solidarity with the victims”, and sent a telegram to Mrs May offering condolences. “We must remain united against these type of threats that affect all of us equally and that know no barriers,” he wrote.
- Turkish President Recep Tayyip Erdogan emphasised that “Turkey feels and shares deeply in the United Kingdom’s pain” and that it stood in “solidarity” with Britain “in the fight against terrorism”. It came just hours after his latest salvo in a fiery row with European allies, especially Germany and the Netherlands, over Turkish referendum campaigning in their countries. Mr Erdogan had warned that Europeans would not be able to walk safely on the world’s streets if “Europe continues this way”.