Prime Minister Theresa May has told MPs: “We will never waver in the face of terrorism.”
Paying tribute to Pc Keith Palmer, who died after being stabbed, she said: “He was every inch a hero and his actions will never be forgotten.”
Delivering a Commons statement she said Wednesday’s attacker was British born and known to police and MI5. The attack was related to “Islamist ideology”.
Labour leader Jeremy Corbyn described the attack as “an appalling atrocity”.
Four people died, including the attacker, with 40 more injured.
Mrs May said the victims included 12 Britons, three French children, two people from Romania, four from South Korea, one from Germany, one from Poland, one from Ireland, one from China, one from Italy, one from the US and two Greek people.
She told MPs: “Yesterday an act of terrorism tried to silence our democracy, but today we meet as normal, as generations have done before us and as future generations will continue to do, to deliver a simple message: ‘We are not afraid and our resolve will never waver in the face of terrorism’.”
- London attacks – latest updates
- London attack: Seven held after Westminster attack
- May condemns ‘sick and depraved terrorist attack’
She said it was still believed that the attacker acted alone and there was “no reason to believe” further attacks on the public were planned.
“His identity is known to the police and MI5 and when operational considerations allow, he will be publicly identified,” she said.
“What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism.
“He was a peripheral figure. The case is historic. He was not part of the current intelligence picture. There was no prior intelligence of his intent or of the plot.”
Describing it as an attack on free people all over the world, Mrs May thanked global allies including US President Donald Trump for their support.
“We meet here in the oldest of all parliaments because we know that democracy and the values it entails will always prevail,” she said.
“Those values – free speech, liberty, human rights, and the rule of law – are embodied here in this place but they are shared by free people around the world.
“A terrorist came to the place where people of all nationalities and cultures gather to celebrate what it means to be free. And he took out his rage indiscriminately against innocent men, women and children.
“This was an attack on free people everywhere – and on behalf of the British people, I would like to thank our friends and allies around the world who have made it clear that they stand with us at this time.”
Mr Corbyn said MPs were “united by our humanity, by our democratic values and by that human impulse for solidarity to stand together in times of darkness and adversity”.
“I express my condolences to the family and friends of PC Keith Palmer, who gave his life yesterday in defence of the public and our democracy – and to the loved ones of those still in a critical condition, including the French schoolchildren visiting our capital from Concarneau in Brittany,” he said.
“The injured include people of ten nationalities. Innocent people were killed yesterday walking across Westminster Bridge as many millions of Londoners and tourists have done before them.”
Earlier MPs, police and workers around Whitehall observed a one minute’s silence. MPs will later pay tribute to the victims of the attack.
Commons deputy speaker Lindsay Hoyle said Parliament’s security committee will hold an emergency meeting later on Thursday to discuss security arrangements on the estate.
Speaking beside a police cordon outside Parliament, Mr Hoyle told BBC Breakfast: “Terrorism will not defeat democracy.
“We’re in a village and our village policeman has been murdered and all of our thoughts are with the family and the other innocent victims.
“But of course the House must continue – we will not give in to terrorism and today we’ll continue.
“We will be paying tributes later this morning and then the House will continue with its business.”
Mr Hoyle, who is also chairman of the security committee, said: “We will be having an emergency meeting. We’ll be getting information – what else needs to be put in place – and we will reflect, not instantly, we’ve got to take on board what’s happened and we will make decisions accordingly. Of course, we’ve got to look after both houses.”
He said there would be support for MPs and staff who “witnessed things they never expected to witness in their lives”, he said.
- Four dead in Westminster terror attack
- ‘Hero’ MP tried to save stabbed officer
- Sturgeon’s ‘solidarity’ with London
- French students hurt in London attack
After MPs observed a one minute’s silence in the Commons, International Trade Secretary Liam Fox opened Commons business to say: “As we begin our questions today, it is appropriate that we recommit ourselves to the values this parliament represents.
“Those who carry out such wicked and depraved actions as we saw yesterday can never triumph in our country – and we must ensure that it is not violence, hatred or division but decency, goodness and tolerance that prevails in our country.”
Theresa May tells Commons: We’ll never waver}