Armed British officers arrested a man carrying knives near Prime Minister Theresa May's office in London on Thursday on suspicion of preparing an act of terrorism.
The 27-year-old man was detained by counter-terrorism officers on Parliament Street, a stone's throw from the parliament building and May's Downing Street office, during a stop-and-search in an ongoing security operation, police said.
No one was injured in the incident, and May was not at Downing Street at the time because she was campaigning in northern England ahead of the June 8 election.
The knife incident came just over a month after a British-born convert to Islam ploughed a car into pedestrians on nearby Westminster Bridge, killing four people, before stabbing to death a police officer in the grounds of parliament.
"Detectives from the Counter Terrorism Command are continuing their investigation, and as a result of this arrest there is no immediate known threat," police said in a statement about Thursday's incident.
"The man ... was arrested on suspicion of possession of an offensive weapon and on suspicion of the commission, preparation and instigation of acts of terrorism. Knives have been recovered from him."
A Western security source said the man, from London, was known to counter-terrorism officers and the domestic intelligence agency MI5 before his arrest. Two hours after his arrest, police reopened streets in the Westminster district, indicating they believed there was no further public risk.
A Reuters reporter and photographer saw police apprehending a dark-skinned, bearded man dressed in a hooded, black coat. After he was detained, the man was stood between two armed police officers with his right hand in a loose bandage.
"I am aware that an individual has been arrested in Whitehall today and that individual has been arrested on the basis of a terrorism charge," May told Sky News.
"Obviously I can't say much more about it because it is an ongoing police investigation but I think it shows that our police and our intelligence and security services are on the alert as they always are looking to keep us safe and secure."
Dramatic pictures of the man's arrest on social media showed a large group of heavily armed officers surrounding the suspect and a Reuters photographer saw police officers standing by three knives lying on the ground.
Police said the suspect was being questioned at a south London police station.
In the wake of the London attack many people have been quick to praise the bravery of the city's "heroes" - the emergency services and members of the public who rushed to help those in need.
Doctors and nurses were seen running from nearby St Thomas' Hospital to help those injured on Westminster Bridge while MP Tobias Ellwood was pictured giving mouth-to-mouth resuscitation to PC Keith Palmer, the officer stabbed by the attacker.
Another of those quickly on the scene outside Parliament was Dr Jeeves Wijesuriya, chairman of the junior doctors' committee at the British Medical Association, who had been talking to a regional BBC team when the events began to unfold.
"I saw the screaming and ran to help," he said.
"The police got me in to help at the scene. We gave the officer cardiac massage and spent 52 minutes trying to resuscitate the other man.
"We did everything we could, in terms of basic life support and starting to triage injured people at the scene."
Image copyrightAPImage captionTobias Ellwood MP (pictured centre with glasses) and Tony Davis (kneeling in blue jacket) were among those who tried to resuscitate PC Palmer
Tony Davis from Gateshead was also among the group who came to PC Palmer's aid.
The Team GB boxing coach and Army veteran had been at the Houses of Parliament for a press event with a group of boxers when he saw "a large chap brandishing two knives come through the gates and start attacking the policeman".
"I had my phone out to take footage for social media about the event. I didn't realise there was a crash then at the time, I thought it was a student demonstration, there were people running, coming round towards the gate," he told ITV's This Morning.
"At that point instinct kicked in, I leaped over the fence because that guy needed assistance... I'm not that specially trained in first aid but I know the basics and can put someone at ease. I put my raincoat underneath to try to stem the blood," Mr Davis added.
"I've obviously reviewed the footage and audio on my phone which I've given to the police. About 90 seconds into it another guy called Mike came and joined in and at this time there were lots of police around, Keith's colleagues. Three and a half minutes was when the MP came in and sort of took over CPR."
"The MP did a tremendous job, everyone there did. I can't commend the air doctor enough. Very professional, very thorough. It was a great show of solidarity.
"I'm not brave, I'm not a hero."
Praise for the police and NHS staff poured in on social media with people posting messages of solidarity and using the hashtag #WeAreNotAfraid.
Dr Tony Joy, one of the London Ambulance team that landed in Parliament Square, said it had been "an awful day and a long night" but "it's a privilege to do the job".
Political campaigner Richard Tice, who witnessed the attack and its aftermath, said the way people on Westminster Bridge helped the injured showed the character of the city.
"The whole length of the bridge there were passers-by tending to the injured before the medics arrived. That's the spirit of London.
"I'm sure that would happen in any city. But that is the bravery and care one human has for another."
Brendan Cox, the widower of MP Jo Cox who was murdered last year, said the attack was a story about "the people that didn't come home yesterday".
He told BBC Radio 4's Today programme: "What helped me in the weeks after (Jo's death) was that sense of public support. Those thousand acts of kindness.
"One of the things we need to be careful about is giving notoriety to the person that did it.
"I would much rather remember the heroes of this story."
Image copyrightGETTY IMAGESImage captionTeams of paramedics, police and air ambulance crews were on the scene within minutes of the attack
Londoners and visitors to the city were encouraged to greet any police officers they saw patrolling the capital on Thursday.
Mayor Sadiq Khan said a candlelight vigil to remember those killed, injured and affected by the attack would be held in Trafalgar Square, not far from where the events happened, on Thursday evening.
His office said: "The Mayor invites all Londoners - and everyone visiting our city - to come together in solidarity to remember those who have lost their lives, to express sympathy with their families and loved ones and to show the world that we are more committed than ever to the values that we hold dear - that we remain united and open.
"London is the greatest city in the world. We will never be cowed by terrorism. We stand together in the face of those who seek to harm us and destroy our way of life. We always have and we always will."
Ms Frade worked at a London college, while Mr Cochran was from Utah, in the US, and visiting the capital with his wife Melissa, who is in hospital with serious injuries.
According to a family statement, the couple were celebrating their 25th wedding anniversary and were due to return to the US on Thursday.
PC Palmer was a 48-year-old father, who was an unarmed member of the Parliamentary and Diplomatic Protection Squad, with 15 years' service in the force.
Seven of the injured are still in hospital in a critical condition. A further 29 had been treated in hospital, police said.
In the attack on Wednesday afternoon, a man drove a car along a pavement on Westminster Bridge, knocking down pedestrians, creating panic and leaving dozens injured.
He then ran towards Parliament where he stabbed PC Palmer. Armed police shot dead the attacker in the grounds.
Mrs May paid tribute to PC Palmer saying: "He was every inch a hero and his actions will never be forgotten."
She also said one of three police officers injured as they returned from an event to recognise their bravery was in a stable condition.
The so-called Islamic State claimed through its news agency that the Westminster attacker was a "soldier of the Islamic State".
Westminster Bridge has now re-opened.
Image copyrightPA/FACEBOOKImage captionPC Keith Palmer stopped the attacker in his tracks. Aysha Frade would be deeply missed by all, the principal at the college said
Mrs Frade worked at a London sixth form college just a few hundred metres from Westminster Bridge.
The principal at DLD College, Rachel Borland, said she was "highly regarded and loved by our students and by her colleagues".
Mrs May said 12 Britons were admitted to hospital and other victims included three French children, two Romanians, four South Koreans, one German, one Pole, one Irish, one Chinese, one Italian, one American and two Greeks.
Thierry Terret, who is in charge of schools in Brittany, said the three injured students were not in a life threatening condition and were on their way home.
An emotional James Cleverly MP asked Mrs May to consider recognising posthumously the "gallantry and sacrifice" made by PC Palmer, who he knew from his time in the Army.
Media captionMr Cleverly fought back tears as he spoke about his friend Keith Palmer
Analysis: The PM's carefully-chosen words
Media captionPrime Minister Theresa May said MPs were meeting as normal to show "we are not afraid"
By Dominic Casciani, home affairs correspondent
The prime minister's carefully-worded statement that the killer was once investigated raises more questions than it currently answers.
It appears that the individual was discounted as a "peripheral figure" on the edge of some other operation. She didn't say whether that means he was considered and discounted for good reason by MI5 intelligence officers, investigated for a criminal offence by detectives or even ever arrested.
However, she also stressed he was not part of the "current intelligence picture" - and that means he wasn't currently on the radar at all. This points to the very difficult dilemma faced by security services combating these kinds of threats.
Every day they have to prioritise, or triage, who to pursue and who to discount. People who were once a threat change their thinking. They grow up, have kids and settle down. MI5, meanwhile, is tasked with focusing on those they know of with the most advanced plans.
Some of those they discount, or temporarily turn away from, later turn out to be more dangerous that initially thought. They include the ringleader of the 7/7 bombings and one of the two men who killed Fusilier Lee Rigby.
Intelligence is never a complete picture - it is not even like a jigsaw with missing pieces. It's a case of trying to interpret fragments of information that rarely amount to a whole.
Westminster attack
22 March 2017
4
people killed
40
injured
12 Britons in hospital
3 French children injured
9 other nationalities affected
8 arrests made
In a statement made earlier outside Scotland Yard, Acting Deputy Commissioner Mark Rowley said seven arrests had been made during raids in London and Birmingham - an eighth was announced several hours later.
The BBC understands that searches and arrests have been carried out in Forest Gate, east London, Wales, Surrey and Sussex, as well as in Birmingham.
"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," Mr Rowley said.
"To be explicit, at this stage we have no specific information about further threats to the public."
A candlelit vigil will be held in Trafalgar Square in central London on Thursday evening
A JustGiving page, set up for the family of PC Palmer, has raised more than £25,000 so far
The flag over the Houses of Parliament is flying at half mast
MPs held a minute's silence before Parliament continued business as normal
The Pope said in a letter to the Archbishop of Westminster that he was "deeply saddened" and "assured the nation of his prayers"
People worried about family and friends can call the police casualty bureau on: 0800 056 0944 or 0207 158 0010. Anyone with images or footage of the incident can send them to ukpoliceimageappeal.co.uk
Image copyrightGETTY IMAGESImage captionA woman lays flowers near the scene of the attackImage copyrightGETTY IMAGESImage captionForensic officers scour Parliament SquareImage copyrightEPAImage captionA photo of PC Palmer is placed among floral tributes on Whitehall
Brendan Cox, the husband of murdered MP Jo Cox, said it was important to remember that "this was a story about people who didn't come home yesterday".
US President Donald Trump was among world leaders to offer their support to Mrs May, tweeting: "Spoke to UK Prime Minister Theresa May today to offer condolences on the terrorist attack in London. She is strong and doing very well."
