Mayor of London Sadiq Khan offered his thanks to the police and emergency services, and said they had showed "tremendous bravery" during today's terrorist attack at Westminster.
One woman was killed, and several people left with "catastrophic injuries" after an incident near the Houses of Parliament this afternoon.
Witnesses reported a car was driven along the pavement on Westminster Bridge, hitting several people, and an attacker then stabbed a police officer. The assailant was then shot by police.
“Today London suffered a horrific attack near Parliament Square which we are treating as a terrorist attack," said Khan.
“A number of people have lost their lives and at least twenty people have been injured.
“My heart goes out to those who have lost loved ones and to everyone who has been affected."
He said: “Tragically, a Metropolitan Police officer who was doing his duty protecting our city is amongst those who have been killed - and my thoughts are with his family this evening.
“I want to express my gratitude on behalf of all Londoners to the police and emergency services who have shown tremendous bravery in exceptionally difficult circumstances.
“I have spoken to the Acting Metropolitan Police Commissioner Craig Mackey, and national lead for Counter-Terrorism Policing and Acting Deputy Commissioner, Mark Rowley - and remain in close contact with them.
“Londoners should be aware that there will be additional armed and unarmed police officers on our streets from tonight in order to keep Londoners and all those visiting our city safe.
“I want to reassure all Londoners and all our visitors not to be alarmed - our city remains one of the safest in the world.
London is the greatest city in the world and we stand together in the face of those who seek to harm us and destroy our way of life. We always have, and we always will. Londoners will never be cowed by terrorism.
Londoners will never be cowed by terrorism. pic.twitter.com/LTLhgZ7OLQ— Sadiq Khan (@SadiqKhan) March 22, 2017
Read more: What the emergency services know so far