There has been an outpouring of sympathy and solidarity with London, following a terror attack which has left at least five people dead.
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German Chancellor Angela Merkel expressed solidarity, saying she was "deeply shocked by attacks on police and members of the public".
"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."
French President Francois Hollande also expressed support, saying: "France knows what the British people are suffering today", following several terror attacks in recent years.
US President Donald Trump called it "big news", offering his "condolences on today's terror attack in London" and "praise for the effective response of security forces and first responders".
"We obviously condemn today's attack in Westminster," said White House press secretary Sean Spicer.
Australian Prime Minister Malcolm Turnbull said the police presence in Canberra would be boosted.
"But Australians should be assured, our agencies are today, as every day, working relentlessly to keep Australians safe."
NATO secretary-general Jens Stoltenberg said all of NATO "stands in solidarity with the United Kingdom, and we stand together in the fight against terrorism".
"Our message to the citizens of the United Kingdom and our colleagues in the British parliament is simple - we stand with you," said Canadian Prime Minister Justin Trudeau, to applause from his own MPs.
New Zealand Prime Minister Bill English tweeted the "thoughts of New Zealand are with the people of London, and with the victims and their family & friends, following this terrible attack".
Dutch Prime Minister Mark Rutte, who recently won an election against an opponent vowing to end immigration, said he was "shocked".
"The British can count on our support. We are monitoring the situation closely and are in contact with the British authorities."
Russian foreign ministry spokesperson Maria Zakharova said they "share their pain".
"We are calling on Russian citizens who are currently in Britain and on the grounds of our embassy in London to show vigilance."
Tel Aviv's city hall lit up in the colours of the Union Jack, the country's deputy Foreign Minister expressing "deep shock".
Spanish PM Mariano Rajot sent his British counterpart Theresa May a telegram.
"An execrable terrorist act like the one that took place today is a reminder that we face complex challenges for the security of our societies," he wrote.
Irish President Michael Higgins expressed sympathy "of the Irish people with the families of those who have lost their lives and to those who have been injured".