UPDATED 9.15PM: Seven people have been arrested and six addresses raided in London, Birmingham and elsewhere in the UK in connection with the London terror attack, Metropolitan Police acting deputy commissioner Mark Rowley says
Rowley said police raided six addresses in connection with Wednesday's attack by a knife-wielding man who also mowed down pedestrians with an SUV.
"We have searched six addresses and made seven arrests. The inquiries in Birmingham and other parts of the country are continuing," he told reporters outside London's police headquarters.
Rowley refused to identify the attacker.
Rowley said it was still his belief the attacker worked alone and was inspired by "international terrorism", adding that there was no specific information to suggest any further threat to the public.
He also revised the death toll down from five to four - the attacker, Pc Keith Palmer, who he stabbed, and a woman in her mid 40s and a man in his mid 50s, who were mown down as the terrorist sped across Westminster Bridge in his car.
He said 29 people had been treated in hospital with seven people in a critical condition.
A doctor who treated the wounded from the bridge said some had "catastrophic" injuries. Three police officers, several French teenagers on a school trip, two Romanian tourists, and five South Koreans were among the casualties.
Speaking outside 10 Downing St. after chairing a meeting of government's emergency committee, COBRA, British Prime Minister Theresa May said that level would not change. She said attempts to defeat British values of democracy and freedom through terrorism would fail.
"Tomorrow morning, Parliament will meet as normal," she said. Londoners and visitors "will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart."
US President Donald Trump was among world leaders offering condolences, and in Paris, the lights of the Eiffel Tower were to be dimmed in solidarity with London.
Wednesday was the anniversary of suicide bombings in the Brussels airport and subway that killed 32 people last year, and the latest events echoed recent vehicle attacks in Berlin and Nice, France.
Conservative lawmaker Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed first aid on the wounded police officer, who later died. About 10 yards away lay the assailant.
"I tried to stem the flow of blood and give mouth to mouth while waiting for the medics to arrive but I think he had lost too much blood," Ellwood said. "He had multiple wounds, under the arm and in the back."
The attack began early Wednesday afternoon as a driver in a grey SUV slammed into pedestrians on the bridge linking Parliament to the south bank of the River Thames.
Former Polish foreign minister Radek Sikorski was in a car crossing the bridge when he heard "something like a car hitting metal sheet" and then saw people lying on the pavement.
"I saw one person who gave no signs of life. One man was bleeding from his head. I saw five people who were at least seriously injured," Sikorski told Poland's TVN24.
Police said one injured woman was pulled from the river.
The car crashed into railings on the north side of the bridge, less than 200 metres from the entrance to Parliament. As people scattered in panic, witnesses saw a man holding a knife run toward the building.
"The whole crowd just surged around the corner by the gates just opposite Big Ben," said witness Rick Longley. "A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can't believe what I just saw."
Daily Mail journalist Quentin Letts said a man in black attacked the police officer before being shot two or three times as he tried to storm into the building.
"As this attacker was running towards the entrance two plain-clothed guys with guns shouted at him what sounded like a warning, he ignored it and they shot two or three times and he fell," Letts told the BBC.