No country has displayed more of a "backslide in democracy" than Greece, the British thinktank Demos has said in a study highlighting the crisis-plagued country's slide into economic, social and political disarray.

Released on the same day that judicial authorities ordered an investigation into a blog posting by a group of reservists in the elite special forces calling for a coup d'etat, the study singled out Greece and Hungary for being "the most significant democratic backsliders" in the EU.

"Researchers found Greece overwhelmed by high unemployment, social unrest, endemic corruption and a severe disillusionment with the political establishment," it said. The report, commissioned by the European parliament, noted that Greece was the most corrupt state in the 28-nation bloc and voiced fears over the rise of far-right extremism in the country.

The report was released as the fragile two-party coalition of the prime minister, Antonis Samaras, admitted it was worried by a call for a military coup posted overnight on Wednesday on the website of the Special Forces Reserve Union. "It must worry us," said a government spokesman, Simos Kedikoglou. "The overwhelming majority in the armed forces are devoted to our democracy," he said. "The few who are not will face the consequences."

With tension running high after a crackdown on the neo-Nazi Golden Dawn party, a supreme court public prosecutor demanded an immediate inquiry into who may have written the post, which called for an interim government under "the guarantee of the armed forces".

The special forces reservist unit whose members appeared in uniform to protest against a visit to Athens by the German chancellor, Angela Merkel said Greece should renege on the conditions attached to an international bailout and set up special courts to prosecute those responsible for its worst financial crisis in modern times. Assets belonging to German companies, individuals or the state should be seized to pay off war reparations amassed during the Nazi occupation.

Underscoring the social upheaval that has followed economic meltdown, the blog post argued that the government had violated the constitution by failing to provide adequate health, education, justice and security.

Insiders said the mysterious post once again highlighted the infiltration of the armed forces by the extreme right. This week revelations emerged of Golden Dawn hit squads being trained by special forces commandos.

Fears are growing that instead of reining in the extremist organisation, the crackdown on the group may ultimately create a backlash. The party, whose leaders publicly admire Adolf Hitler and have adopted an emblem resembling the swastika, have held their ground in opinion polls despite a wave of public outrage. Golden Dawn, which won nearly 7% of the vote in elections last year and has 18 MPs in Athens' 300-member parliament, has capitalised more than any other political force on Greece's economic crisis. "Much will depend on how well it will withstand the pressure and they are tough guys who seem to be withstanding it well," said Giorgos Kyrtsos, a political commentator.