A man is being held for allegedly attempting a terror attack after being arrested near the Houses of Parliament.

The Metropolitan Police said the 27-year-old suspect was stopped by armed officers in Parliament Street for a search “as part of an ongoing operation”.

“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,” a spokesperson added.

“Knives have been recovered from him. He is being detained under the Terrorism Act and is in custody in a south London police station.

Scotland Yard said the investigation had been handed to its Counter Terrorism Command and that there was “no immediate known threat” to the public.

The man, described as "very calm" by a witness, was said to have been carrying a rucksack containing at least three knives when he was detained shortly before 2.30pm on Thursday.

Police reportedly followed him before the arrest as he travelled on the London Underground.

Dressed in dark clothing, the suspect 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.

He is understood to have been living in south-east London but was born abroad.

A man is detained by police officers near Downing Street (Getty Images)

Handcuffed, with his arms behind his back, one of his hands appeared to be wrapped up in a bandage as four officers stood next to him.

Held near a red telephone box, the suspect had his black hood up as he was led away.

Whitehall, which houses several government departments, was temporarily shut off by police after the incident.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

The road was quickly reopened to traffic after witnesses saw the suspect being handcuffed and taken away by armed police.

The man was detained yards from Parliament, where a suspected Isis supporter killed five people in a car and knife rampage last month.

Khalid Masood was shot dead after killing four people on Westminster Bridge before crashing his car and stabbing PC Keith Palmer dead at the entrance to the House of Commons.

Isis claimed responsibility for the attack, following atrocities across Europe in response to its calls for followers to target "disbelievers" in the West.

Prime Minister Theresa May was away from Number 10 on a campaign visit at the time of the latest incident, which did not disrupt work in Downing Street, a spokeswoman said.

A spokeswoman said: "The Prime Minister has been informed by officials and she will be updated, as usual for incidents like this."

Eyewitnesses Harry Peasnell, 27, and Andy Burnage, 22, were leaving the adjacent building which houses Government offices when they were told over the tannoy the usual entrance on to Whitehall was closed.

Ali’s knives and rucksack could be seen on the ground after his arrest ( (Reuters)

As they made their way back towards Parliament from the alternative Horse 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 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.