But MPs did not need to take cover - the 250-strong mob are extras in a major film being shot on the estate.
Suffragette, which tracks the struggle for women to get the vote, stars British favourite Carey Mulligan and features Meryl Streep as fearsome campaigner Emmeline Pankhurst.
It is the first time the historic venue has been used for commercial filming, after MPs agreed it was a good way to cover costs.
Scenes have also been shot in Central Lobby and a committee room, but not in the Commons or Lords chambers.
With MPs away on their Easter break, the usual ministerial cars were replaced by veteran vehicles and an angry mob of placard-wielding women squaring up to police officers on horseback.