By Adam Fleming
BBC political correspondent
The Heathrow airport expansion protest puts the security procedures in Parliament under the spotlight again.
Protesters unfurled banners on the roof of Parliament
The protesters from Plane Stupid would have passed through airport-style metal detectors.
The campaigners say they took a lift to the top floor of the building and walked out on the roof, where they unfurled their banners. The visitors' passes stuck to their jackets were clearly visible on the television pictures of the protest.
A team of Metropolitan Police officers provide security for Parliament.
It is also overseen by two officials - the Serjeant-at-Arms and Black Rod. The Joint Committee on Security examine security procedures in the long term.
Security in Parliament was increased in 2004 after some high profile breaches. Campaigners from Greenpeace scaled Big Ben.
Protesters threw purple powder at Tony Blair during Prime Minister's Question Time. Pro-hunt campaigner Otis Ferry and seven other men stormed the Commons chamber.
After a review security cameras were installed and a glass screen was put around the public gallery.
In several of those cases the protesters had been escorted into the building by pass-holders. The speaker Michael Martin reminded MPs that they could be suspended if their guests did not behave properly.
Recently concrete barriers have been placed around Parliament to defend the building from attacks using vehicles. MPs and officials must swipe their security passes to gain access to certain areas.
Liberal Democrat MP Norman Baker says the Palace of Westminster is a difficult building to secure: "It's not one new modern building which is easy to police.
"It's a combination of different buildings linked by corridors, by underpasses and by ad-hoc arrangements bolted on over the years."
The challenge for Commons authorities has been to tighten security while ensuring that visitors can see democracy in action - it has always been a fundamental part of British democracy that the public can have access to Parliament and their elected representative.
Correction 23 July 2008: This story has been amended to correct the description of hunting protester Otis Ferry.