David Cameron and the British government were labelled "America's digital pit bull" on Tuesday at the launch of a campaign by writers against mass surveillance.

British author Priya Basil told a press conference in Berlin that the political reaction to the revelations from whistleblower Edward Snowden had been "worse in the UK than in Germany".

"You can really feel that our democracy and our freedom of speech are under threat," said Basil, one of a small group of international writers who organised the project. "That's why I hope this appeal will have even more resonance in places like Great Britain." More than 500 of the world's leading authors, including five Nobel prize winners, have issued a statement condemning the scale of mass surveillance revealed by whistleblower Edward Snowden and warning that spy agencies are undermining democracy.

The signatories, who come from 81 different countries and include Margaret Atwood, Don DeLillo, Orhan Pamuk, Günter Grass and Arundhati Roy, say the capacity of intelligence agencies to spy on millions of people's digital communications is turning everyone into potential suspects, with worrying implications for the way societies work.

They have urged the United Nations to create an international bill of digital rights that would enshrine the protection of civil rights in the internet age.

At Tuesday's press conference, several new authors – including Italian writer Umberto Eco and British writer William Boyd – were unveiled and organisers said they hoped people around the world would now join the campaign by signing up through the website change.org.

"What Snowden has started with a little ball, we will hopefully turn into the beginning of an avalanche," Danish author Janne Teller told the Guardian. However, she said the campaign should not be seen as targeting governments, but as a move to help them.

"The mass surveillance, secret services and defence industry are far beyond the American government and any of our governments and are a power themselves. This initiative must be seen as helping governments, who like to preserve democracy in the western world. It's not against government, but the abuse of power."

Over the past six months, the Guardian, along with some of the world's leading media outlets, has published a number of stories detailing the mass surveillance techniques of the British intelligence agency, GCHQ, and its US counterpart, the NSA, following leaks from former NSA contractor Snowden.

Two of the most significant programmes uncovered were Prism, run by the NSA, and Tempora, which was set up by GCHQ. Between them, they allow the agencies to harvest, store and analyse data about millions of phone calls, emails and search-engine queries.

The authors' statement does not mention these programmes by name, but it states that the extent of surveillance revealed by Snowden has challenged and undermined the right of all humans to "remain unobserved and unmolested" in their thoughts, personal environments and communications. "This fundamental human right has been rendered null and void through abuse of technological developments by states and corporations for mass surveillance purposes," the statement adds.

"A person under surveillance is no longer free; a society under surveillance is no longer a democracy. To maintain any validity, our democratic rights must apply in virtual as in real space."

At Tuesday's conference Basil said she hoped the movement would encourage people around the world to take a stand. "I think if I was just in the UK, I'd feel so disheartened, but I think part of this worldwide appeal is also for people to know it works differently in different places and it's possible to go further than you think."