A suspected terrorist attack was foiled after armed police arrested a man who is alleged to have been found carrying knives near the Houses of Parliament.
The Guardian understands the operation was triggered following a tip-off to police by a member of Britain’s Muslim community who was concerned about the man’s behaviour.
Officers were lying in wait for the suspect whose movements were being tracked by a joint police and MI5 operation as he travelled into central London, swooping close to Westminster underground station.
The scene where the man, aged 27, was arrested is metres away from where Khalid Masood launched his attack on Westminster bridge and the parliamentary estate five weeks ago, leaving five people dead, including himself. Masood used a vehicle to mow down pedestrians before using a knife to stab a police officer to death.
The daylight arrest followed a tense intelligence operation following the tip-off from someone who knew the suspect, with further investigation identifying the man detained as being of concern to counter-terrorism investigators. .
The suspect in the latest incident was seen pinned to the ground after his arrest at 2.22pm as armed officers stood over him. Shortly afterwards forensic experts could be seen examining the ground where two knives had fallen on to the road, as did a bag, believed to have contained the knives.
The man is believed to be from the London area but was born outside the United Kingdom.
The suspect is believed to have been acting alone, and the investigation will try to establish what his targets may have been . One source said the stop was “not random, it was targeted”.
Given the location of the arrest, investigators believe one possibility was an attack on the Westminster or Whitehall areas. Past Islamist propaganda has called for attacks on civilians or symbols of power. Security is especially tight around Whitehall following last month’s attack .
Police said the arrest came at the junction of Parliament Street and Parliament Square, barely 30 metres from Westminster Bridge.
Witnesses said the suspect wrestled to the floor by police but appeared calm after his detention.
MI5 and Scotland Yard’s counter-terrorism command will try to establish if the suspect had any direction from overseas, and whether from Islamic State played any role – either directly, through contact, or indirectly through its propaganda urging attacks.
The motivation of the suspect will be a key part of the investigation, and his mental health will be of keen interest to investigators. At least two knife attacks thought initially to be terrorist acts, were later determined to be best explained by the suspects’ severe mental health problems.
Police said in a statement: “A man has been arrested in Whitehall this afternoon, at approximately 14.22, following a stop and search as part of an ongoing operation. The man was arrested in Parliament Street … by armed officers from the Met’s specialist firearms command.
“The man, aged in his late 20s, 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 on Parliament Street for more than half an hour, without speaking or struggling. While two officers detained him, around two dozen heavily armed police blocked off the traffic and pedestrians on one side of the road with tape.
He was led away into a police car at around 3.15pm, while scene-of-crime officers examined knives and a rucksack on the traffic island in the middle of Parliament Street at the junction with Parliament Square.
The prime minister, Theresa May, praised the work by police and MI5, saying: “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.
“I would say that we owe a huge debt of gratitude to these people, many of whom 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.”
Features of the arrest were highly unusual. Police prefer to arrest terrorism suspects in dawn raids at their homes, away from any potential target of attack. One source said there may have been a need to gather evidence to secure a conviction. Arresting someone in their home with knives poses challenges of convincing a jury that a terrorist attack was planned.
Harry Peasnell, 27, and Andy Burnage, 22, were leaving a building adjacent to the scene which houses various government offices after a meeting with officials when they were told the usual entrance on to Whitehall was closed. As they left the area via an alternative route 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. Peasnell 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.
Witness Ian Moss, who was travelling on a bus through Whitehall at the time, said the suspect 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, Moss said: “No, not at all. He had police pointing arms at him though.”
A French tourist who declined to be named, said: “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 incident unfolded with few MPs present in the Palace of Westminster because parliament is winding down before the election and May has already headed out on to the campaign trail. However, civil servants were still working as normal, and a number of HMRC staff were locked outside their building when the Whitehall entrance was closed off.
The atmosphere in parliament remained calm and all gates were operating as normal during the incident. No injuries were reported.
The nervousness around security at Westminster was heightened when, in a separate incident, packages addressed to parliamentarians containing white powder were intercepted before they could reach their victims.
The terrorism threat level for the United Kingdom remains at severe, meaning an attack by terrorists is assessed to be highly likely. The biggest threat is seen as coming from Islamic State, which constantly urges followers to stage attacks.