London: British police have made eight arrests and searched six addresses after an attack near the British Parliament which left four people dead, including the attacker, and at least 29 injured.
Police and MI5 were hunting for friends and associates of the attacker, named as Khalid Masood, behind Wednesday's atrocity in Westminster, in a race to discover whether he was a 'lone wolf' or part of a broader terror cell.
In a speech to the House of Commons, Prime Minister Theresa May said the attacker was British-born and his identity was known to British police and to MI5.
"It is still believed he acted alone, the police have no reason to believe there are imminent further attacks", May says.
"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 the plot.
"Our working assumption is that the attacker was inspired by Islamist ideology."
Police later named him as 52-year-old Khalid Masood, who was born in Kent and had recently lived in the West Midlands.
Associated Press reported that Islamic State, through its Aaamaq news agency, had claimed the London attacker was "a soldier of the Islamic State."
The chaotic incident saw the British Parliament on lockdown for hours and the political heart of the British capital brought to a standstill.
On Thursday morning Assistant Commissioner of Police and Head of Counter-terrorism Mark Rowley revealed that police had raided six addresses and made seven arrests as part of their investigation, which covered London, Birmingham and other places. The number of arrests was later updated to eight.
Hundreds of detectives had worked through the night on the case.
He said police believed - which was being borne out by the continuing investigation, that the attacker "was acting alone and inspired by international terrorism".
Police had no specific information about further threats to the public.
However enquiries, searches and arrests were continuing, and investigators were still working to establish the attacker's motive, preparation and associates.
He asked that the media not publish the name of the attacker at a "sensitive stage of the investigation".
He also said seven people injured in the attack remained in hospital in a critical condition.
Two members of the public died on Westminster Bridge as the attacker mounted the pavement at 2.20pm on Wednesday and sent tourists and locals "flying like footballs", as one eyewitness described it.
The four people killed include a woman in her 40s, a man in his 50s, the police officer and the attacker, police confirmed on Thursday.
Twenty-nine people were treated in hospital for injuries, including police officers. One woman who fell into the Thames was rescued and treated for serious injuries.
The injured are believed to include five South Koreans, three French schoolchildren, and one German woman living in South Australia, who received surgery in hospital for a foot injury.
After mowing down pedestrians, the driver smashed into the railings of parliament and tried to enter.
However not far inside the grounds he was confronted by an unarmed policeman, Keith Palmer, 48, whom the attacker fatally stabbed.
Armed officers then shot the intruder dead.
A Conservative MP, Foreign Office minister Tobias Ellwood, tried to resuscitate the policeman. Mr Ellwood, a former army officer who lost his brother in the Bali attacks in 2002, was hailed a hero after his selfless act in running towards the site of the attack to try to help the PC.
Just hours after the terror attack on Westminster Bridge, police raided a flat in Birmingham - a move widely reported as linked to the terror attack, though police declined to comment "for operational reasons".
Dozens of police joined the raid, cordoning off several streets.
According to reports, several men were taken into custody. BBC Newsnight said police were led to Birmingham after tracing the rental of the Hyundai i40 car used in the attack.
More police action is expected on Thursday, in a massive operation involving the police counter-terror command and intelligence service MI5.
The murdered policeman was a husband and father, a member of the Parliamentary and Diplomatic Protection Service. He had been a police officer for 15 years.
Assistant Commissioner Rowley said Mr Palmer "was someone who left for work today expecting to return home at the end of his shift, and he had every right to expect that would happen".
He added that police were assuming the attacker's motive to be "Islamist related".
Late in the evening British Prime Minister Theresa May gave a defiant speech outside Number 10 Downing Street, where she pledged life would go on in the capital in spite of the "sick and depraved terrorist attack".
"We will come together as normal," Mrs May said. "And Londoners - and others from around the world who have come here to visit this great City - will get up and go about their day as normal.
"They will board their trains, they will leave their hotels, they will walk these streets, they will live their lives.
"And we will all move forward together. Never giving in to terror. And never allowing the voices of hate and evil to drive us apart."
The Queen has cancelled a visit to New Scotland Yard which had been planned for Thursday.
Extra police have been ordered onto the streets of London as a precaution, as a temporary measure. Westminster Tube station, the closest to the attack, is closed.
The nation's terrorism threat level remains at severe - where it has been since mid-2014 - meaning another attack is "likely". There is only one higher level, when an attack is believed "imminent".
Mrs May said it was "no accident" that the attacker had chosen Westminster, which she said was engrained with a spirit of freedom, democracy and human rights that made it a target for those who reject these values.
