London Police today said they have made a total of eight arrests as part of counter-terrorism investigation into the London attack.
British Prime Minister Theresa May told the Parliament that eight arrests have been made in police searches overnight.
Meanwhile, Britain's top anti-terror officer Mark Rowley said police have revised down the number of victims from Wednesday's rampage to three from four. Some 40 people were injured.
Rowley said the raids included locations in London and the central city of Birmingham.
The authorities worked round-the-clock to piece together what happened as the attacker ran down several pedestrians on the nearby Westminster Bridge then charged at a policeman at the Parliament gates, stabbing him to death with a large knife.
Armed officers shot the attacker dead but not before he killed two members of the public and the 48-year-old policeman.
Press Association news agency photos believed to be of the attacker lying on an ambulance stretcher showed a burly man wearing black clothes and having a beard.
Other pictures showed two people being treated on the ground inside the vehicle entrance gates of Parliament, with a knife visible on the cobblestones, while three shots were heard ringing out on video footage as terrified passersby fled.
Parliamentarian Mary Creagh told AFP there was "a real sense of panic" as the attack unfolded and a doctor at nearby St Thomas' Hospital said they were treating people with "catastrophic" injuries.
'Sick and depraved'
Prime Minister Theresa May described the attack as "sick and depraved" in a defiant address in which she affirmed Parliament would meet as normal today.
Standing outside her Downing Street residence after an emergency Cabinet meeting, May said Britain's alert level would remain unchanged at level four, or "severe".
''We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart,'' said May, dressed in black.
The Prime Minister was in Parliament at the time of the attack and was ushered away in a silver car as gunfire rang out.
Rowley said on Wednesday police suspected they were dealing with ''Islamist-related terrorism'', adding that investigators believed they knew the assailant's identity.
Queen Elizabeth II postponed her appearance on Thursday to open the new headquarters of London's Metropolitan Police, where the force's flag flew at half-mast after the attack.
The attack came a year to the day after Islamic State jihadists killed 32 people in twin bomb attacks in Brussels and after a series of deadly assaults in Europe that had spared Britain - until Wednesday.
Parliament was locked down for several hours and police evacuated MPs and visitors to nearby Westminster Abbey and the Metropolitan Police headquarters.
An air ambulance flew in and police cordoned off a large area, while tourists on the London Eye, a popular visitor attraction, were left dangling up to 135 metres (443 feet) in the air for more than an hour during the attack.
Foreign Office Minister Tobias Ellwood, whose brother Jonathan was killed in the 2002 Bali bombing, was pictured with his face smeared with blood helping to give first aid to the fatally wounded police officer.
The 2005 attack
The worst previous attack in London was in 2005 when four suicide bombers, inspired by Al-Qaeda, targeted the transport system, killing 52 people.
Britain's last terror attack was last year's assassination of MP Jo Cox by a pro-Nazi sympathiser in her constituency in northern England.
Britain's allies reacted with shock and vowed to stand with London in the fight against terror while lights on the Eiffel Tower in Paris were switched off at midnight in solidarity with victims of the attack.
US President Donald Trump and French President Francois Hollande both spoke to Ms. May and Chancellor Angela Merkel said Germany stood with Britons "against all forms of terrorism".
''Spoke to UK Prime Minister Theresa May today to offer condolences on the terrorist attack in London,'' Trump tweeted.
The incident dominated Britain's front pages, with The Mirror describing it as an "attack on democracy" while London's Evening Standard carried the headline "Terror carnage at Westminster".
Several international tourists visiting one of London's most iconic sights were caught up in the violence.
Five South Korean tourists were injured, Seoul's foreign ministry said, while the Romanian government said two of its citizens were also injured.
A Portuguese man was hurt, the country's government said, while a seriously injured woman was rescued from the Thames following the incident.
French Foreign Minister Jean-Marc Ayrault said he would travel to London to visit three French pupils on a school trip who were among those hurt.