The high-profile measure is proof of the red-alert security status at a time when Britain's top policeman believes a terror attack on London is inevitable.
It symbolises the changing face of democracy, which is gradually removing high-profile politicians from contact with the public.
The move follows intelligence warnings of a plot to attack Tony Blair with deadly anthrax or sarin gas.
Police are also worried terrorists could smuggle a plastic gun through the security cordon or release a chemical weapon in the Commons.
They want to protect the Prime Minister from an assassination attempt because since September 11 he has become one of the three world leaders most at risk.
Security has already been tight in Downing Street and the route his motorcade takes when it travels to the Commons is now varied each time.
Large concrete blocks have been placed around the Palace of Westminster to prevent suicide vehicle attacks.
Police snipers watch constantly from surrounding buildings, security for all visitors has been stepped up with airport-style metal detectors and body searches, and there are armed officers at all entrances. And the authorities are considering more measures, which could include a 15ft wall topped with jagged spikes around the palace.
But a confidential security review in Parliament concluded the X-ray machines are not sophisticated enough to prevent a terrorist getting into the chamber with new hi-tech liquid or plastic explosives.
MPs are getting an extra week's holiday at Easter to give contractors time to install the bullet and blast-proof glass screen in front of historic Strangers' Gallery.
Visitors to the Commons will watch proceedings from behind the screen.
Some senior politicians fought a year-long rearguard action against the plans, fearing it would be a literal barrier between them and the public and showing the growing lack of trust.
But concerns were heightened during February's Commons debate on the Hutton Report when a series of anti-war protesters were ejected from the public gallery. In previous incidents in the 1970s and 1980s demonstrators hurled teargas canisters, paint and even three bags of manure at MPs.
And a group of lesbians abseiled into the chamber from the public gallery, screaming abuse at then Prime Minister Margaret Thatcher.
While members of the public are scanned a second time before entering the Commons chamber, the Commons authorities decided on more security.
The original plans for a huge screen had to be scaled down because it would have taken too long to install.
The authorities do not comment formally on security matters but have been reviewing them since September 11. It emerged the Palace of Westminster and especially Big Ben were seen by terrorists as prime targets because of the site's prestige and world fame.
The security of Parliament was further placed in question a fortnight ago when two anti-war demonstrators, brothers Harry and Simon Westaway, were able to scale the fences and climb the Big Ben clock tower.