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BREAKING: Scotland Yard now says five people have died, and at least 40 are injured, after this morning's terror attack in London's Westminster.
That includes three members of the public, the attacker himself, and a police officer - a husband and father who'd served for 15 years. Police are not commenting on the attacker's identity but they're working on the assumption is was inspired by international terrorism.
British Prime Minister Theresa May has called the overnight terror attack outside the Houses of Parliament in London "sick and depraved".
"The location of this attack was no accident," she said. "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."
Prime Minister Malcolm Turnbull has declared Australia's "heartfelt sympathy and resolute solidarity" is with the people of the UK.
"The attack on the British Parliament is an attack on parliaments, freedom and democracy everywhere," he said.
"We send our condolences, especially to the families of the victims, those injured, including a police officer murdered by the terrorist as he attempted to enter the Houses of Parliament."
The PM confirmed there'll be an increased police presence at Parliament House in Canberra today. "But Australians should be reassured, that our agencies are today as every day working relentlessly to keep Australians safe," he said.
"We will never ever let the terrorists win. We will defeat and destroy them on the battle field, we will defeat and defy them at home."
The heart of London has been sent into lock-down after a terrorist attack where four people died including the attacker, and at least 20 others injured, some of their injuries described as "catastrophic".
A man allegedly driving a four-wheel-drive into crowds on Westminster Bridge, killing two, before crashing the vehicle into a railing and trying to get into parliament.
It's here he fatally stabbed an armed police officer before being shot dead by another.
"Sadly I can confirm that now four people have died, that includes the police officer protecting parliament and one man we believe to be the attacker...the officer's family has been made aware. At least 20 people have been injured," said Acting Deputy Police Commissioner Mark Rowley.
It's believed the attacker was acting alone, but the area around parliament has still been sent into lock-down as a thorough search continues.
No Australians are thought to be caught up in the attack. At this stage the travel warning for the UK has not changed: Smart Traveller's telling Australians to exercise normal safety precautions, but also to remain vigilant, listen to media reports, and follow the advice of local authorities.
US President Donald Trump has also made contact with British PM Theresa May with the White House condemning the attack.