The House of Commons has been re-opened after being "locked down" after a person was seen throwing powder around in the building's central lobby.
All doors and exits were closed with no-one allowed in or out of the building while the incident was investigated.
There were fears the powder could have been dangerous but MP Angus Robertson said it was thought to be wheat flour.
Business continued as usual inside the Commons during the half-hour alert.
But anti-terrorist police were sent to the scene to investigate.
Nick Assinder, the BBC News website's political correspondent was 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 lock-down.
"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 - this had happened on at least one previous occasion, following the purple powder attack on Tony Blair during question time by members of Fathers for Justice.