- Video report by ITV News Security Editor Rohit Kachroo
A man carrying a bag of knives has been arrested in Whitehall on suspicion of planning a terror attack, police have said.
The arrest came following a stop and search, the Metropolitan Police said, adding the 27-year-old had at least three knives.
It is thought the man was known to intelligence services prior to the arrest which was carried out by MI5 and counter-terror police.
- ITV News Security Editor Rohit Kachroo explains what is known so far about the arrest:
The suspect - described as "very calm" by a witness - was wrestled to the ground on a traffic island at the Parliament Square end of Whitehall, close to the Foreign Office and the entrance to Downing Street.
The arrest was made just yards from the scene of the terror attack by Khalid Masood which left five people dead.
- Watch footage from central London
No injuries were reported from the scene, although the man arrested appeared to have one hand wrapped up in a bandage as police arrested him.
Images from the scene showed three knives and a brown backpack.
Armed police closed the main gates to Parliament and traffic was restricted outside the road leading to Downing Street - the residence of Theresa May and ministries.
Theresa May was not in Downing Street at the time of the arrest, but was campaigning in Derbyshire ahead of the General Election.
Speaking from Derbyshire Mrs May said she was "aware" of what had happened in Whitehall and praised the "bravery" of the police, intelligence and security services.
The Prime Minister continued: "It shows that our police, and our intelligence, and our security services are on the alert as they always are, looking to keep us safe and secure.
"We owe a huge debt of gratitude to these people, many of who are unseen, unheard, yet the job they do day in, day out to keep us secure is a really important one and we should thank them for it."
In a statement, the Metropolitan Police said the arrest was made following a stop and search.
"A man has been arrested in Whitehall this afternoon, at approximately 2.22pm, following a stop and search as part of an ongoing operation.
"The 27-year-old man was arrested in Parliament Street, junction with Parliament Square, by armed officers from the Met's Specialist Firearms Command.
"He 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.
"He is being detained under the Terrorism Act and is in custody in a south London police station.
"Detectives from the Counter Terrorism Command are continuing their investigation, and as a result of this arrest there is no immediate known threat."
Harry Peasnell, 27, a recruiter who often attends meetings in Westminster, said police were "quick on the scene" and that he was not worried like he was during the terror attack in the area just weeks ago.
"Last time all the buildings were on lockdown and no one was allowed to leave," he said, adding that on Thursday people were allowed to leave the buildings.
Westminster has been on a state of heightened alert since Khalid Masood launched an attack on March 22, killing five people, including Pc Keith Palmer.
Masood drove at pedestrians before stabbing Pc Palmer as he guarded the cobbled forecourt of the Palace of Westminster. Masood was shot dead by police.
A French tourist, who was in New York at the time of the 9/11 terror attacks and was visiting London with her daughter, saw the arrest happen.
The witness, who did not want to be named, said: "I was here half an hour ago. We just saw the man arrested and that's all.
"He was very calm. I think they got him on the ground, then they stood him up against the wall.
"There were lots of police officers. The road was blocked.
"We didn't stay here, we just moved because I was a bit afraid because of my daughter.
"We saw the helicopter was here so we came back to see what was going on.
"I was in New York on September 11, so I am a bit scared of these things and being in the wrong place at the wrong moment."