London's mayor said the city "will never be cowed by terrorism" following a vehicle and knife attack that killed four people, including a police officer, near Parliament on Wednesday.
"Today London suffered a horrific attack near Parliament Square which we are treating as a terrorist attack," Mayor Sadiq Khan said in a statement.
"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," he said. "We always have and we always will. Londoners will never be cowed by terrorism."
Two people were killed after an assailant drove a 4x4 vehicle into a crowd on Westminster Bridge, and a police officer died after being stabbed during the attack which began at around 2:40 p.m. local time (10:40 a.m. ET), authorities said. Police later announced another death, bringing the number of victims killed to four.
The attacker, who police believe is the sole suspect, was shot dead by authorities. He has not been identified. London Metropolitan Police Acting Deputy Commissioner Mark Rowley said authorities believe they know the attacker's identity, and are working under the assumption it was Islamist-related terrorism.
British Prime Minister Theresa May called it a "sick and depraved terrorist attack" and said Parliament would meet as normal Thursday morning. She said the attacker drove into a crowd of pedestrians, then ran towards Parliament, where he attacked an officer before being killed.
"The values our Parliament represents — democracy, freedom, human rights, the rule of law — commands the admiration and respect of free people everywhere," May said Wednesday evening. "That is why it is a target for those who reject those values."
"But let me make it clear today, as I have had cause to do so before, any attempt to defeat those values through violence and terror is doomed to failure," she said.
The country's terror threat level was already at "severe" and would not change because of Wednesday's attack, May added.
Approximately forty people were injured in the attack, police said. Twelve suffered serious injuries and were transported to hospitals, London Ambulance Service Deputy Director of Operations Pauline Cranmer said. Others were treated at the scene.
Three police officers were among those injured in the vehicle attack on pedestrians on the bridge, May said. She hailed the bravery of emergency and security services.
Khan said that more armed and unarmed police officers would be on the streets to protect London.
May vowed that Londoners would continue to live their lives as normal. "We will all move forward together — never giving in to terror, and never allowing the voices of hate and evil to drive us apart," she said.