Prime Minister Theresa May has confirmed that four people - including one police officer and the perpetrator - died in today's "sick and depraved" attack in Westminster.
Speaking outside 10 Downing Street after chairing a meeting of the Government's Cobra emergencies committee, Mrs May paid tribute to the "exceptional men and women" of the police force who responded to the attack.
She confirmed that a single attacker drove a car into pedestrians walking on Westminster Bridge, killing two of them, before running with a knife through the front gates of the Palace of Westminster, where he killed a police officer before being shot dead.
She vowed that "the forces of evil" would never be allowed 'to drive us apart.'
Mrs May said that any attempt to defeat the values that Parliament stands for was "doomed to failure".
And she vowed: "We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
The UK's terror threat level would remain at 'severe', Mrs May said.
She praised the bravery of the police officers 'who keep us and our democratic institutions safe'.
"Our thoughts and prayers go out to all who have been affected, to the victims themselves and to their families and friends, who waved their loved ones off but will not now be welcoming them home," she said.
"For those of us who were in Parliament at the time of this attack, these events provide a particular reminder of the exceptional bravery of our police and security services who risk their lives to keep us safe.
"Once again today, these exceptional men and women ran towards the danger, even as they encouraged others to move the other way."
Mrs May added: "That they have lost one of their own in today's attacks only makes their calmness and professionalism under pressure all the more remarkable.
"The location of this attack 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.
"These streets of Westminster, home to the world's oldest parliament are ingrained with a spirit of freedom that echoes in some of the furthest corners of the globe.
"And the values our Parliament represents - democracy, freedom, human rights, the rule of law, command the admiration and respect of free people everywhere.
"That is why it is a target for those who reject those values.
"Let me make it clear today, as I have had cause to do before, any attempt to defeat our values through violence and terror is doomed to failure."
She said Parliament would 'meet as normal' on Thursday.
"We will come together as normal and Londoners and others from around the world who have come here to visit this great city will get up and go about their day as normal."
Mrs May said that the UK's security threat level, which has stood at "severe" for some time, will not be raised as a result of the incident.
The exact details of the 'sick and depraved terrorist attack on the streets of our capital this afternoon' were still emerging, she said, adding: "Having been updated by police and security officials, I can confirm that this appalling incident began when a single attacker drove his vehicle into pedestrians walking across Westminster Bridge, killing two people and injuring many more, including three police officers.
"This attacker, who was armed with a knife, then ran towards Parliament, where he was confronted by the police officers who keep us and our democratic institutions safe.
"Tragically, one officer was killed. The terrorist was also shot dead.
"The United Kingdom's threat level has been set at severe for some time and this will not change.
"Acting Deputy Commissioner Rowley will give a further operational update later this evening."
The 45-minute Cobra meeting brought together senior ministers as well as top police officers, representatives of the security agencies and London Mayor Sadiq Khan.
Mrs May ordered flags to be lowered to half mast over Downing Street as a mark of respect to the innocent people who lost their lives at Westminster.
Among ministers attending this evening's meeting were Defence Secretary Sir Michael Fallon, Security Minister Ben Wallace, Transport Secretary Chris Grayling, Health Secretary Jeremy Hunt and Foreign Minister Sir Alan Duncan.
Absent were Home Secretary Amber Rudd, who was away on an official trip to Pakistan, and Foreign Secretary Boris Johnson, in the US for a summit of nations involved in the fight against the Islamic State terror group.
The meeting in Whitehall was addressed by Metropolitan Police Assistant Commissioner for Specialist Operations Mark Rowley, Britain's most senior anti-terrorist police officer.