Tunisia's president is using a referendum on a new constitution to move back towards dictatorship, the head of the biggest Tunisian opposition party, the Islamist Ennahda, warned on Friday.

Rached Ghannouchi, who was parliament speaker until President Kais Saied shut the chamber and seized most powers last summer, said the atmosphere in the build-up to the July 25 vote was "undemocratic and funereal".

Saied's focus on political changes while the economy slid further towards crisis also meant Tunisia faced a "Sri Lanka situation" with a looming social explosion, Ghannouchi told Reuters in an interview.

"Saied's speeches can only be translated into clashes, chaos and assassination... Tunisia could reach the point of famine and is in dire need of dialogue to confront these crises," he said.

Ghannouchi, a former political prisoner, has been a leading player in Tunisian politics since the 2011 revolution that ousted autocratic leader Zine al-Abidin Ben Ali, introduced democracy and triggered the Arab Spring uprisings.

He has been a main critic of Saied since the president's seizure of broad powers last year.

"Tunisia is moving towards a dictatorial regime that brings together all powers (under one person)," he said.

Saied has said his actions were needed to save Tunisia from years of political infighting and stagnation, and that his constitution is correcting the course of the 2011 revolution.

This year, in the run-up to the referendum, Saied has greatly expanded his control over the judiciary and the formerly independent electoral authority, raising questions over rule of law and a fair vote.

His proposed constitution gives him nearly unlimited powers while diluting the role of the parliament and judiciary. Ennahda and most other parties have urged a boycott of the referendum.

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Next week, Ghannouchi must appear before an investigating judge on suspicion of corruption, which he and Ennahda both deny, calling the charges political.

Activists have warned that the authorities may use his hearing to arrest Ghannouchi. "It would not be surprising if I am arrested," he said.

"Dictatorial regimes seek to use the judiciary and employ the security services, as Ben Ali did, against the opposition," he said.

Ghannouchi compared the new constitution, which contains a legislative body based on regional councils, to that of Libyan dictator Muammar Gaddafi and other Arab autocrats.

"The constitution establishes a unilateral, populist regime reminiscent of that of Gaddafi and the Arab dictators," he said.

However, he said, democratic principles had become well established among Tunisians over the period since the revolution. He said Ennahda was urging its supporters to boycott the referendum instead of voting 'no' because "entering a game where your opponent sets all the rules is unwise".

Ennahda will call protests for July 19, the day he faces the judicial hearing, and July 23, two days before the referendum, he said.