World leaders stand in support of Britain after London terror attack

World leaders have unanimously condemned the London terror attack and vowed to stand with Britain in the face of extremism.

At least four people were killed and more than 40 injured in the attack outside parliament Wednesday (local time), when a man mowed down pedestrians with a car then leapt out and fatally stabbed a police officer.

The assailant was himself shot dead by police.

Read more: Police officer among five dead, Australian woman injured in Westminster rampage

In an address outside her Downing Street office, Prime Minister Theresa May described the attack as "sick and depraved" saying the assailant chose the site as an assault on Britain's democratic values.

Popular UK monuments Big Ben and the London Eye lit up in red and blue soon after the attack.

The library of Birmingham was also illuminated in the colours of the Union Jack.

In France, the lights of the Eiffel Tower in Paris were switched off.

French President Francois Hollande sent a message of "solidarity" and "support to the British people", stating his country could identify with their pain after the attack, in which three French high school students were also wounded.

"France, which has been hit so hard in recent times, knows what the British people are suffering today," he said.

And Prime Minister Bernard Cazeneuve tweeted: "Solidarity with our British friends, horribly attacked, full support for the injured French students, their families and their friends."

European Commission President Jean-Claude Juncker said the attack left him "highly emotional".

In Brussels to commemorate the one-year anniversary of attacks on the country’s main airport and a metro station, Mr Juncker said "the fact that exactly on the same day something similar happened in London, and to London, is really putting me in the situation of someone who does not have... enough words to express how I am deeply feeling."

Read more: Belgium falls silent one year after Brussels attacks

Meanwhile, Australian Prime Minister Malcolm Turnbull said the assault was an attack on "freedom and democracy everywhere".

"But we will never, ever let the terrorists win. We will defeat and destroy them on the battlefield, we will defeat and defy them at home," he said.

"We will never change the way we live. We will never let them divide us."