A man armed with several knives has been arrested in Whitehall on suspicion of planning a terror attack, Scotland Yard said.
The suspect was carrying a rucksack containing at least three knives when he was detained "following a stop and search as part of an ongoing operation", police said.
The Metropolitan Police said in a statement that he was held at approximately 2.22pm on Thursday by armed officers from the Specialist Firearms Command.
It went on: "The man, aged in his late twenties, 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."
The suspect was held yards from the scene of last month's terror attack by Khalid Masood.
The bearded suspect, who was dressed in dark clothing, 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.
Images from the scene showed three knives and what appeared to be a mobile on the ground close to a brown rucksack.
An officer in forensic overalls could be seen taking photographs of the knives.
The suspect was detained at the scene for several minutes within view of journalists and passers by before he was eventually taken away in a police vehicle.
Recruiters Harry Peasnell, 27, and Andy Burnage, 22, were leaving the adjacent building which houses various Government offices after a meeting with officials when they were told over the tannoy the usual entrance on to Whitehall was closed.
As they made their way back towards Parliament from the alternative House Guards Road entrance they said they were pushed back up Great George Street by several plain clothes police officers.
They then made their way to Parliament Square where they said they saw a man in his 20s or 30s, who they thought was the suspect, being spoken to by police next to the wall of the offices.
Mr Peasnell, who says he regularly has 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.
Westminster has been on a state of heightened alert since Masood launched his 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. He was shot dead by police.
A builder working on the road nearby said: "I saw the police officers catch him. They caught him and got him on the ground."
Asked if the suspect was waving knives, acting aggressively or shouting, the witness said: "No."
Eyewitness Ian Moss, who was travelling on a bus through Whitehall at the time, said the suspect - who police said is 27-years-old - was on the ground in the middle traffic island.
He said police "looked like they were tending" to him, adding: "He was arrested. He could walk so no idea if he was injured."
Asked if he was struggling while on the ground, Mr Moss said: "No - not at all. He had police pointing arms at him though."
A French tourist, who was in New York on 9/11 and was visiting London with her daughter, saw the arrest happen.
The witness, who declined 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 policemen. 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."
Prime Minister Theresa May was away from Number 10 on a campaign visit at the time of the incident, which did not disrupt work in Downing Street, said a spokeswoman.
"This is an ongoing police operation and we are monitoring it," said the Number 10 spokeswoman. "The Prime Minister has been informed by officials and she will be updated, as usual for incidents like this."