A major review of security is under way after condoms full of purple flour were thrown at Tony Blair as he faced MPs in the House of Commons.
Mr Blair was replying to the Tory leader when the dust was thrown
The PM was speaking during his weekly half-hour question and answer session when one of the projectiles hit his back, prompting an evacuation by MPs.
Campaign group Fathers 4 Justice claimed responsibility. Two men aged 50 and 36 have been arrested.
It is believed the suspects got tickets to the Commons at a charity auction.
The incident occurred despite a £600,000 security screen being installed in the chamber at Easter.
Father-of-two Ron Davies, from Worthing, West Sussex, threw the flour-filled
condom from an area of the gallery reserved for MPs' and Peers' guests and notable visitors as a second activist, named by police sources as Guy Harrison, held up a poster.
A spokesman for Fathers 4 Justice said Mr Harrison was from Steyning, West Sussex.
Deputy Prime Minister John Prescott and Chancellor Gordon Brown, who were sitting either side of Mr Blair, both looked up at the public gallery as the dust descended at 1218 BST.
MPs were seen wafting something away from themselves as proceedings were suspended.
Government sources admitted that the politicians did not know what to do and expressed surprise that people were let out of the chamber.
The source said the MPs' response "did not go the way we expected". The attack would have been "incredibly serious" if the dyed flour had been anthrax or ricin.
In a later statement to the House of Lords, Labour peer Baroness Golding said the two protesters were guests of hers and she offered "unreserved apologies" to Commons Speaker Michael Martin, MPs and fellow peers.
The protest could result in more security restrictions in Parliament
Matt O'Connor, spokesman for Fathers 4 Justice, said two members of the group were involved in the incident to highlight MPs' failure to help fathers gain access to children through the courts.
Mr Blair's spokesman said no sooner had the prime minister left the Commons, he wanted to know when he could return to finish question time - but the Speaker vetoed the move.
Instead Mr Blair began preparing his speech for a seminar on alcohol abuse on Thursday, said the spokesman.
The prime minister appeared unsure of what was happening to him before he was ushered slowly, almost bemused, from the chamber
He added the security breach had "serious implications", but stressed that it was up to the Commons to resolve these.
Home Secretary David Blunkett said a security review had already been launched at Westminster after recent breaches at Buckingham Palace.
The security services will bring forward recommendations "very quickly indeed", but it could mean there will be more restrictions on access to Parliament, he said.
"Thank God it has not led to danger and fatality," Mr Blunkett added.
Labour MP Kevin Brennan said he saw what appeared to be a stain on the back of Mr Blair's jacket as the dust hit him.
Ex-Tory minister Michael Portillo said he thought "one dose" of the dust had been thrown "before the prime minister reacted".
Commons leader Peter Hain said: "This was a serious incident. I have asked for an immediate report on the circumstances and what additional security provisions may be necessary."
Mark Oaten, the Liberal Democrats home affairs spokesman, called for an urgent review of security arrangements.
"He threw what looked like a paint ball at the prime minister and shouted Fathers 4 Justice," he said.
WESTMINSTER SECURITY SCREENING:
Glass screen across public gallery, but does not cover area where MPs or peers' guests sit
"At that point the prime minister remained remarkably calm, I thought, if not a bit confused and the Speaker quite rightly suspended the session.
"Police were then seen rushing into the chamber to try to arrest the individual. There was absolutely no panic at all."
For the Conservatives David Davis said: "This incident is profoundly disturbing in the current security situation."
Prime Minister's questions was abandoned. Commons business recommenced at 1330 BST.
HOUSE OF COMMONS POWDER ATTACK
1: Projectiles thrown by two protesters in section of gallery above Commons reserved for ambassadors, members of the House of Lords, their guests and MPs' guests.
2: Purple powder sprays over Labour backbenchers on benches below.
3: Two projectiles reach Tony Blair, standing at the government despatch box. One hits his back, the other hits the floor at his feet.
4: Speaker Michael Martin suspends proceedings.