Within minutes of this afternoon’s attacks, police had thrown a wide cordon around the Palace of Westminster, as police and ambulance sirens shrieked towards parliament and an air ambulance landed outside.

The lockdown was so swift and sudden that a red, double-decker bus trapped inside the cordon had to back out towards Westminster Abbey as police roared at onlookers, reporters and cameramen to move back.

Inside the parliamentary estate, MPs and staff were "quietly stunned", according to Ulster Unionist Party MP Danny Kinahan.

“We all know we’re in a fairly secure location and we probably assumed because it is the Houses of Parliament that it was always going to be a target but you just don’t expect it to happen here. But those who witnessed things were I think also stunned, they were shocked,” he told The Irish Times.

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“The bells had just gone for a vote and we, the MPs, were all heading down to the chamber to vote, to be told that there was a lockdown and proceedings were being suspended, and get back to your office. And on the way back to the office we then could hear outside police shouting to someone to get down on the ground and lie down and sort of military organisation and police organisation going on.”

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Those MPs already in the chamber were told to remain there as proceedings were suspended. As police helicopters flew overhead, white-clad forensics experts moved into the palace and police prepared to search every room in the parliamentary buildings.

As news filtered through to MPs about the stabbing of a police officer and the shooting of an alleged assailant, some questioned how such a breach could happen in one of the most heavily secured locations in the capital.

Mr Kinahan, a former army officer, was more sanguine, praising the speed with which the police responded. “I think we’ve got to praise the police and so on for their efficiency and our sympathy with anyone who has been injured or hurt,” he said.

Most MPs were initially unaware of the incident outside parliament, when a vehicle crashed into the perimeter fence after mowing down a number of pedestrians on Westminster Bridge. Eyewitnesses crossing the bridge reported seeing bodies lying on the ground after the attack, which happened moments before shots were heard inside the grounds of parliament.