Hundreds of people were evacuated from Parliament to Westminster Abbey.
Addressing the crowd, which included ministers, MPs, peers and tourists, from the pulpit a Met Police superintendent said it would take around 90 minutes to process everyone.
An email address and website was given out so eyewitnesses can upload any pictures or videos of what happened for use in evidence.
Police said people would be assessed to see if they have any information.
A prayer was then said for the victims of the attack.
Metropolitan Police Chief superintendent Raj Kohli addresses the crowd from the pulpit at Westminster Abbey to some of the hundreds of people that was evacuated from Parliament during the terrorist attack. (PA)
MPs including Health Secretary Jeremy Hunt, along with parliamentary staff, were also held in Westminster Hall - the oldest part of the Palace.
They were told by police that it could be up to 90 minutes before they would be allowed to leave as officers sought witnesses and mobile phone photos and footage.
The people assembled in the hall were told that officers were also taking care not to disturb the crime scene outside.
Around 1,000 people who were taken to Westminster Abbey for safety are now being processed by police.
Scotland Yard urged the crowd to put delays in letting them leave into context.
Chief Superintendent Raj Kohli told the crowd: "There are families planning funerals as we speak so let's put it into context."
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