British Prime Minister Theresa May says the deadly Westminster attack was a "sick and depraved" assault on a parliament which stands for democracy and freedom.
World leaders have sent messages of sympathy and support following the attack near the the Houses of Parliament in London which left five people dead].
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Mrs May said the location of the attack, home to the world's oldest parliament, was no accident.
"The terrorist chose to strike at the heart of our capital city where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech."
She praised the "exceptional bravery of police and security services" who ran towards danger as they encouraged people to move the other away.
The UK's threat level had been set at severe for some time and this would not change, she said. Parliament will be open as normal tomorrow.
Leaders react to terror attack
Referring to the "terrible attack", New Zealand Prime Minister Bill English said the thoughts of the country were with the people of London and the victims.
The thoughts of New Zealand are with the people of London, and with the victims and their family & friends, following this terrible attack.— Bill English (@pmbillenglish) March 22, 2017
Mr English said innocent people should not have to fear such violence when going about their daily lives and that New Zealand utterly condemned "this shocking attack".
"London is a place many thousands of New Zealanders have visited and called home, and where many more have friends and family based, so this attack feels very close to home," Mr English said.
He said New Zealand stood alongside the UK in the fight against terrorism and violent extremism.
In London, mayor Sadiq Khan released a video statement expressing gratitude to police and emergency services and said Londoners would never be cowed by terrorism.
"London is the greatest city in the world and we stand together in the face of those who seek to harm us and destroy our way of life."
Londoners will never be cowed by terrorism. pic.twitter.com/LTLhgZ7OLQ— Sadiq Khan (@SadiqKhan) March 22, 2017
US President Donald Trump, in a call to Mrs May, offered his condolences and pledged "the full cooperation and support" of the US government, the White House said in a statement.
European leaders were also swift to condemn the attack and express solidarity with Britain.
German Chancellor Angela Merkel in an emailed statement said: "Although the background to these incidents still needs to be exactly clarified, I confirm on behalf of Germany and its citizens that we stand firmly and resolutely by Britain's side when it comes to fighting any kind of terrorism."
European Commission president Jean-Claude Juncker said: "Together we will never cease to prove that love triumphs over hate, tolerance over fear."
Spanish Prime Minister Mariano Rajoy condemned the attack and said Spain was with the British people.
French President Francois Hollande said: "Terrorism concerns us all, and France knows what the British people are suffering today."
Just days ago, a man was shot dead after grabbing a soldier's gun at Paris' Orly airport, the latest in a series of attacks.
Mr Hollande added in a video statement that a response to the attacks must be made "at the European level, and even beyond that".
Canadian Prime Minister Justin Trudeau said: "Our thoughts are with the victims of today's attack in London and their families. Canadians remain united with the people of the UK."
Russia called the attacks evil.
"We don't split terrorism into categories; we consider it as absolute evil," Foreign Ministry spokeswoman Maria Zakharova said.
"At this moment, as always, our hearts are together with the British people, we feel their pain and speak again about the need to confront that evil."