Members of the Fathers-4-Justice activist group threw purple flour bombs at Prime Minister Tony Blair on Wednesdy during his weekly question-and-answer session at Parliament.
The projectiles left a purple powder to rain down on lawmakers while one 'bomb' even hit the PM on the shoulder.
Purple is the international colour for equality, a Fathers-4-Justice spokesman said - promising numerous more publicity stunts before Father's Day on 20 June.
The group is comprised of divorced fathers who seek to bring attention to their struggle for equal rights of access to children.
But the prank led to the suspension of debate and an anti-terror squad was called in to test if the dust thrown from the upper gallery was noxious.
Some parliamentarians said Blair's life could have been at risk but he was unruffled by the protest, a senior aide said.
Parliament resumed after a break of about 70 minutes.
Parliament was the target of a major security breach in March when two men protesting against the Iraq war scaled its landmark Big Ben clock tower.
Blair was also heckled at question-time in February by anti-war protesters.
* Picture coutesy of parliamentary recording unit