Stephen Fry has denounced the government's failure to act over the mass surveillance programme revealed by the whistleblower Edward Snowden, labelling its behaviour as "squalid and rancid".

Opening a day of debate to galvanise action against spying by the British and US intelligence agencies, Fry said that exploiting the fear of terrorism is a "duplicitous and deeply wrong means of excusing something as base as spying on the citizens of your own country".

The performer was speaking via a prerecorded interview at a London summit on Saturday marking the anniversary of the start of Snowden's revelations, which were first published in the Guardian and the Washington Post.

The day of action is billed as the biggest privacy event of 2014, with more than 500 people attending the event at Shoreditch Town Hall in east London.

In his video message, Fry, 56, said: "The idea of having your letters read by somebody, your telegrams, your faxes, your postcards intercepted, was always considered one of the meanest, most beastly things a human being could do, and for a government to do, without good cause. Using the fear of terrorism that we all have, the fear of the unknown that we all share, the fear of enemies that hate us, is a duplicitous and deeply wrong means of excusing something as base as spying on the citizens of your own country."

The broadcaster said GCHQ and NSA had cooperated to "read and intercept everything we send".

"It's enough that corporations know so much about us and our spending habits, our eating habits, our sexual preferences, everything else," he said.

"But that a government, something that we elect, something that should be looking out for our best interests, should presume without asking to take information that we swap, we hope privately, between ourselves is frankly disgraceful."

More than 500 people attended the event at east London's Shoreditch Town Hall, organised by the Don't Spy on Us Campaign, a coalition of privacy, free expression and digital rights organisations which is urging the UK government to end the mass surveillance of the web and mobile phone networks by the British eavesdropping centre, GCHQ.

Among the speakers was Wikipedia founder Jimmy Wales, who said: "The tide is beginning to turn as the public comes to understand just how broken the surveillance state is."

Other high-profile speakers included Alan Rusbridger, the Guardian editor-in-chief, who led the team that masterminded a series of remarkable disclosures from the files leaked by the National Security Agency whistleblower.

Before a packed audience, Rusbridger recounted the "frenetic" period last summer when the British government attempted to strong-arm the Guardian into not publishing further revelations, at one point forcing senior editors to destroy hard-drives holding some of the encrypted files leaked by Snowden. However, he said that the British government's heavy-handed reaction backfired.

"By forcing the reporting out of the UK to the USA, the British government lost any handle on this story at all. So, I hope that the British government will think about that in the future."

Rusbridger also lamented the UK's lack of an enshrined right to free speech, referring to the US constitution's first amendment and the "quasi-constitutional role" of that nation's press.

"We need to embody some of those rights here, we don't have rights in Britain. We tend to wait until things go wrong, so there is no really established right to privacy because there is no constitutional protection of free speech," he told the audience.

Rusbridger added that those keen to "shut down" the Snowden story tended to frame it as a question of privacy versus national security.

However, he said, Snowden's revelations opened up many more fundamental issues of public interest.

The Don't Spy on Us Campaign is calling for an inquiry that will report before next year's general election and investigate the extent to which current laws have failed to protect the privacy of the public. In addition, it is demanding new legislation that will make the security agencies properly accountable to MPs.

Other measures include the use of judges instead of the home secretary to decide when spying can be justified.

Blogger and activist Cory Doctorow said: "Freedom from surveillance is essential to freedom itself. The freedom to think, to speak and to have discourse without fear of reprisal or even judgment is at the core of democracy itself."

Emma Carr, acting director of Big Brother Watch, called on the government to publicly acknowledge that the UK's surveillance laws need to be reviewed.

"Without affirmative action, the government will certainly find that the general public's faith in politicians to properly monitor how the security agencies are using surveillance powers will diminish," she said.

Gos Hosein, executive director of Privacy International, added: "Secret surveillance is anathema to a democratic society, as no real debate can take place without an informed public."