The House of Commons has been re-opened following a lockdown after a man was seen throwing powder around in the building's central lobby.
The Commons was locked down for about half an hour
All doors and exits were closed with no-one allowed in or out of the house for about half an hour, and the man was led away to a police van in handcuffs.
There were fears the powder could have been dangerous but MP Angus Robertson said it was thought to be wheat flour.
The Tories said the incident showed more must be done to improve security.
The party's homeland security spokesman, Patrick Mercer, said that things had improved considerably since the Fathers for Justice powder attack last year, but extra measures must be taken.
He admitted that plans to locate screening areas for visitors underground would be 'expensive and delayed', but insisted that they were needed.
"Maintaining a degree of security is just a nightmare if you wish to allow voters to move around in a democracy," he said.
BBC political editor Nick Robinson said the man shouted about "corruption" while he was being led away by police.
It is thought the man believed himself to have been corruptly defrauded of his living by a High Court judge, our correspondent added.
It was initially feared the substance thrown could have been anthrax.
The lobby was sealed off and officers wearing protective overalls, gas masks and gloves were seen entering the area, before the all-clear was given and the Commons re-opened.
Parliamentary business continued as usual inside the Commons during the half-hour alert.
Anti-terrorist police went to the scene.
Earlier journalist Nick Skeens told BBC News the powder had remained on the floor of the lobby with police around it.
He said: "It must have been about six o'clock because we'd gathered for an exhibition for an initiative called Joined-up Design for Schools when apparently somebody threw a lot of white powder on the ground.
"The police then moved very swiftly into action and moved people out of the way and stopped us from leaving.
"Now I understand they've brought in some forensic experts to test it and it's been declared okay. We can walk past it".
Nick Assinder, the BBC News website's political correspondent, was also in the building during the alert.
He said: "At about 1815 BST anyone attempting to leave the palace found all the doors barred after police subjected the entire place to a lockdown.
"We were simply told there had been a security alert and we would not be allowed to leave until the incident had been checked out.
"It only later emerged that this was apparently an individual in central lobby, which is open to the public, who had been involved in an incident.
"There was no sense of panic in Westminster as the security staff attempted to get to the bottom of the incident."
Assinder said it was not the first time the Palace of Westminster had been shut down.
Since the purple powder attack on Tony Blair during question time by members of Fathers for Justice, there has been at least one previous occasion of a lockdown, he said.