The letter signatories claimed that Poland is heading towards authoritarianism, adding that society and opposition parties needed to mobilise and work out a common plan to defend democracy.
But the letter has been slammed by senior figures in the conservative Law and Justice (PiS) government as “absurd.”
The letter was signed by former presidents Lech Wałęsa, Aleksander Kwaśniewski and Bronisław Komorowski, by former foreign ministers including Włodzimierz Cimoszewicz and Radosław Sikorski, as well as by former communist-era oppositionists.
Prime Minister Beata Szydło said: “Poles chose Law and Justice in parliamentary elections, they chose PiS’s programme and gave us responsibility for conducting Polish affairs. This was the free choice of Poles."
Deputy Prime Minister Piotr Gliński told private broadcaster TVN24 that the language of the letter was “absurd.”
He added: “It is absurd to claim that democracy in Poland in under threat."
Referring to the signatories of the letter, the head of the Prime Minister's Office, Beata Kempa, said: "I advise these gentlemen to come to terms with the results of the [October 2015] election" in which PiS came to power in a landslide victory.
"It’s the nation, not a few men, who decide who governs the country," Kempa added.
The letter warned that an ongoing row over Poland’s Constitutional Tribunal “is a realistic threat to [Poland’s] membership rights in the EU being limited. Anti-European and xenophobic statements and actions by those in power undermine the cohesion of the Union, serving the interests of an imperialist Russia.”
It added: “The foundations of our security and economic development are being shaken. "
The letter accuses Law and Justice of “usurpation of power” adding that “those guilty of violating the constitution will bear responsibility."
Poland is locked in a political stalemate after the conservative Law and Justice party, which came to power in October, introduced sweeping reforms to the Constitutional Tribunal and other institutions.
Earlier this month, the European Parliament passed a resolution warning that the “effective paralysis” of Poland's Constitutional Tribunal endangers the rule of law, democracy and human rights.
The tribunal has itself rejected PiS-backed changes to the way it functions. PiS, in turn, has refused to recognise that ruling by the tribunal, claiming it is invalid.
Critics say the PiS-backed changes were designed to paralyse the tribunal, which decides whether laws passed by parliament are in keeping with the Polish constitution.
PiS has argued it is unfair that a tribunal with a majority of judges appointed under the previous parliament should be able to scupper flagship policies for which the party secured a mandate in democratic elections.
In January, the European Commission said it was starting a probe into whether laws pushed through by Law and Justice violate EU standards. (pk)