Londoners have come together at a vigil in an attempt to show the city's defiance in the face of terror and to "show solidarity".
London Mayor Sadiq Khan, Metropolitan Police Acting Commissioner Craig Mackey and Home Secretary Amber Rudd spoke in Trafalgar Square to tell terrorists they will not win.
It was just over 24 hours after British-born attacker Khalid Masood fatally wounded three people on Westminster Bridge before getting into the concourse of Parliament where he stabbed a police officer to death.
Masood, 52, who was born in Kent but had recently been living in Birmingham, was then shot dead by armed officers.
Islamic State has claimed Masood was one of their "soldiers".
Mr Khan told a square that was packed with people: "Those evil and twisted individuals who tried to destroy our shared way of life will never succeed and we condemn them.
"When Londoners face adversity we always pull together. We stand up for our values and show the world we are the greatest city in the world."
Ms Rudd said: "They will not win, we are all connected and we showed that today, by coming together, by going to work, by getting about our normal business, because the terrorists will not defeat us, we will defeat them."
Dr Omer El-Hamdoon, deputy secretary general of the Muslim Council of Britain, said: "For any person to attack innocent people is outrageous and despicable and we condemn it unreservedly.
"Unfortunately, some people will use this to carry on spreading hate and sometimes we see a symbiotic relationship between the far-right and people like ISIS (another name for IS) who feed off each other to spread hate.
"We have to work together to not allow terrorism to divide us."
Forty people were injured in the atrocity, with 29 being treated in hospital, seven of whom remain in a critical condition.
Mr Mackey, who was actually leaving the Houses of Parliament at the time through the gate that Masood used to enter, said it was a "truly terrible" incident.
He said: "This cannot be undone, much as we would wish it.
"However we do get to choose our reaction and gathering here tonight shows exactly how we must move forward. We must stand together.
"People have tried to tear this city apart with acts of terror many times before. They have never succeeded and they never will."
Many of the thousands who attended the vigil echoed the words of the officials in explaining why they had come to a spot that police had been warning people to stay away from the day before.
One, a young woman, told Sky News: "It could have been anybody on that bridge.
"We all have to stick together and show support and solidarity. They might have scared us but they won't beat us."
Another, a man who said he was from the Muslim community, added: "This is our home. You've got to stand together.
"When you get such a person as this - I cannot even express what sort of brain he had. This is time to show we are together.
"Love for all; hatred for none - that is our message. The word Islam means peace. Going out and doing this sort of atrocity is ... (the) opposite of Islam."
A teenage boy who was at the vigil with his mother said: "I wanted to show that we will stand together and we are a strong community and that terrorism isn't a religion. It's just a bad bad thing."