Terror group Islamic State on Thursday claimed responsibility for the terrorist attack near the Parliament on Wednesday afternoon in which four people were killed including the lone attacker.
The knife-wielding attacker had first plowed an SUV into pedestrians on Westminster Bridge, killing two and injuring over 30 before he fatally stabbed a police officer on Parliament grounds. Police shot and killed the attacker, whose identity they have not yet disclosed.
Aaamaq news agency, the media arm of the ISIS, said the London attacker was a "soldier of the Islamic State."
It added that the person "carried out the operation in response to calls for targeting citizens of the coalition."
IS has called on its supporters to carry out attacks against citizens of the U.S.-led coalition that has been targeting the group since 2014.
Police in Britain have arrested at least seven people in connection with the attack.
British Prime Minister Theresa May today delivered a defiant message a day after an alleged terrorist attack near the Parliament on Wednesday afternoon. Addressing the House of Commons, May said that Britain would not be cowed down by such acts.
May today said that police know the identity of the British-born attacker. He was once investigated for extremist links but was considered a peripheral figure, May said, without disclosing his name.
Police believe the man acted alone and there is no reason to believe "imminent further attacks" are planned, May said.
Seven people were arrested in raids, including some in the city of Birmingham as police searched for clues.
May told lawmakers in the House of Commons that "yesterday an act of terrorism tried to silence our democracy, but today we meet as normal."
She called the car and knife rampage that killed three victims "an attack on free people everywhere."
Britain's Parliament observed a minute of silence to remember a police officer and two civilians killed in the attack.
Scotland Yard Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley said the investigation was at a critical stage and the identity of the attacker was not being released as the ongoing investigation tries to piece together the suspects "motivations, his preparation and associates".
Rowley said that he believed the attacker acted alone and was "inspired by international terrorism."
"The inquiries in Birmingham, London and other parts of the country are continuing. It is still our belief - which continues to be borne out by our investigation - that this attacker acted alone and was inspired by international terrorism. To be explicit, at this stage we have no specific information about further threats to the public," he said.
Rowley confirmed that the victims were a mix of nationalities, and included a woman in her 40s and a man in his 50s.
He revised the death toll from five to four, including the attacker, the police officer and two civilians.
"The latest figures I have are that there are currently four dead and 29 people were treated in hospital. The fourth man of course was the terrorist who was shot dead by armed police at the scene. We are still collating numbers of walking wounded and of those in hospital sadly seven of them are in a critical condition," he said.
Overnight, West Midland Police officers stormed a second-floor flat in the city believed to be the residence of the knife-wielding attacker and led away people in handcuffs.
West Midlands Police referred all enquiries about the incident to the Metropolitan Police.
The attacker drove a car at top speed before stabbing a police officer at the gates of Parliament and being shot dead by Scotland Yard officers.
It has also emerged that the 4x4 car which the terrorist suspect used to ploughed down pedestrians on Westminster Bridge was reportedly hired from Solihull area of Birmingham.
London Mayor Sadiq Khan called for Londoners to attend a candlelit vigil at Trafalgar Square on Thursday evening in solidarity with the victims and their families and to show that London remains united.
The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack was "highly likely."
Defense Secretary Michael Fallon told the BBC that authorities assume the attack was linked to "Islamic terrorism in some form." Investigators worked around the clock.
"They have been working right through the night, looking into his background, how he got hold of the vehicle, where the vehicle has been in the last day or two, and who may or may not have helped him," he said.
Speaking outside 10 Downing St. after chairing a meeting of government's emergency committee, COBRA, Prime Minister Theresa May said Wednesday that level wouldn't change. She said attempts to defeat British values of democracy and freedom through terrorism would fail.
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," May said.
President Donald Trump was among world leaders offering condolences.
London has been a target for terrorism many times over past decades. Just this weekend, hundreds of armed police took part in an exercise simulating a "marauding" terrorist attack on the River Thames.
(With AP inputs)