The heads of GCHQ, MI5 and MI6 have claimed that the work of their organisations has prevented 34 UK terror plots since the 2005 London bombings.

GCHQ director Sir Iain Lobban, head of MI5 Andrew Parker, and head of MI6 Sir John Sawers all faced questions from the Intelligence and Security Committee (ISC), in a 90 minute televised public hearing yesterday.

The unprecedented decision to make the hearing public was a move the committee chairman Sir Malcolm Rifkind called a "very significant step forward in the transparency of our intelligence agencies".

Of the 34 terror plots they claim to have prevented, Parker claimed that "one or two" would have resulted in mass casualties. These statistics justified the £2 billion cost of the agencies and answered the criticisms about mass surveillance, which they have faced since leaks by former American National Security Agency (NSA) contractor Edward Snowden came out earlier this year.

"The suggestion that what we do is somehow compromising freedom and democracy - of course we believe the opposite to be the case," Parker said. "The work we do is addressing directly threats to this country, to our way of life and to the people who live here."

Earlier this week Sir Tim Berners-Lee, creator of the world wide web, condemned mass internet surveillance by GCHQ and the NSA, calling their attempts to break encryption "appalling and foolish".

Glenn Greenwald, the Guardian journalist who helped bring the revelations to light, criticised the ISC before the hearing yesterday but believes that real accountability can still be achieved.

"I think the system has failed to exercise meaningful accountability up to this point because there was a huge suspicionless system of mass spying that the British and American people had no idea had been built in their name," Greenwald said. "But I think that system can bring about real accountability if there is the political will."