Turkey's Nobel laureate Orhan Pamuk has said that the situation in his country "is going from bad to worse and even towards terrible" following the government's attempts to block access to Twitter, as a phalanx of major writers, from Zadie Smith to Günter Grass, line up to state their "grave concern" about "the freedom of words" in Turkey today.

The authors, who also include Salman Rushdie, Margaret Atwood, Karl Ove Knausgaard, and Grass and Pamuk's fellow Nobel laureate Elfriede Jelinek, have added their names to a joint letter from PEN International and English PEN which calls last week's ban on Twitter "an unacceptable violation of the right to freedom of speech". The Turkish government restricted access to the micro-blogging website, and prime minister Recep Tayyip Erdoğan indicated the ban could be extended further, saying he would not "leave this nation at the mercy of YouTube and Facebook" and pledging to "take the necessary steps in the strongest way".

On Thursday, he appeared to follow up that threat as Turkish media reported that the country's telecommunications authority (TİB) had blocked YouTube as a "precautionary administrative measure". The latest curbs came hours after an audio recording of a high-level security meeting was leaked on the video-sharing website. Turkey previously banned YouTube in 2007, but lifted the ban three years later.

According to PEN, the Twitter ban currently remains in place despite an administrative court order on 26 March ordering its suspension pending a court judgment.

Pamuk, who won the Nobel in 2006, not long after Turkish state prosecutors had charged him with "insulting Turkishness" for remarks he made to an interviewer, told the Guardian "there were times in Turkey when free speech was equally bad … as in 2008 when YouTube was closed. But at that time we were at least hopeful about the future. Not now. The situation here is going from bad to worse, and even towards terrible."

Pamuk's translator Maureen Freely, the new president of English PEN, added: "The real worry, I think, is that Erdoğan now seems to have the power to do whatever he wishes, whenever he wishes. And that is why we're so concerned about what he'll do next, to silence dissenters and crush opposition wherever he sees it."

The signatories to the letter point to the fact that Turkey ranks 154th among 180 countries on the World Press Freedom Index, saying that even today, "translators, editors, publishers, poets and writers face criminal proceedings and even imprisonment for legitimate expression under a variety of legislative fetters". PEN particularly highlighted the case of Muharrem Erbey, a human rights lawyer and writer who was arrested in 2009, and who is currently in prison awaiting trial. PEN believes the charges against the author are politically motivated, and that Erbey is being held because of his alleged affiliation with Kurdish political parties. The free speech organisation is calling for his immediate release.

In their letter, the writers – from Turkey and around the world – are asking Turkey's leaders "not to retreat from democracy and its keystone, freedom of speech; but rather to recognise their obligations under international treaties and to lift the block on Twitter with immediate effect".

'Democracy in Turkey is sorely lacking' … Turkish writer Elif Shafak was tried for 'insulting Turkishness'

"We urge them to remember that this beautiful country will be stronger and happier when, and if, it appreciates pluralism, diversity and the freedom of words," they write. "We welcome the administrative court in Ankara's decision to suspend the [Twitter] ban ahead of a full judgment and urge the telecommunications authority to restore access to Twitter immediately."

The Turkish author Elif Shafak, who was also tried for "insulting Turkishness" following the publication of her novel The Bastard of Istanbul, is one of the authors to sign the letter. "Turkey's rulers need to understand that democracy is not solely about getting a majority of votes in the ballot box. Far beyond that, democracy is a culture of inclusiveness, openness, human rights and freedom of speech, for each and every one, regardless of whichever party they might have voted for. It is the realisation of the very core of democracy that has been sorely lacking in Turkey today," said Shafak, whose trial – based on words uttered by fictional characters in her novel – went on for a year before charges were dropped.

The letter comes in the wake of a new report from PEN International and English PEN on the Gezi Park protests last year, and their impact on freedom of expression. "The excessive use of police force, as well as widespread media censorship and reprisals against journalists and users of social media, starkly illustrated the shortcomings of Turkish democracy in its lack of pluralism and disregard for fundamental rights and freedoms," says the report. It also lists the "human rights violations" that took place during the protests and calls on the Turkish government to, among other things, ensure freedom of expression is respected and peaceful protestors are not "arbitrarily detained". PEN International and English PEN also wants the right to peaceful assembly included in Turkey's proposed new constitution.