Tunisia was plunged into crisis by the July 25 killing of opposition MP Mohamed Brahmi, an attack blamed on radical Islamists, and mediators hope the long-awaited talks will bring an end to the political paralysis that has since the country.
"Ennahda and some other parties are engaged in positive preparations (for the planned dialogue), and then there are negative preparations taking place that aim to destroy the consensus," said Noureddine Aarbaoui, vice president of Ennahda's political bureau.
"One side calls for dialogue and the other for protests," Aarbaoui told radio station Shems FM on Tuesday.
The talks are due to begin at 1400 GMT on Wednesday at the human rights ministry. Only representatives of political parties have been invited, with neither the prime minister nor the president expected to attend, according to the powerful UGTT trade union, the chief mediator in the crisis.
But a coalition of secular opposition parties is planning demonstrations in Tunis on Wednesday to demand the Ennahda-led government's immediate departure just as the national dialogue is finally due to begin.
Aarbaoui called on the UGTT to persuade the opposition parties to abandon their planned rally.
"We need a clear process of dialogue and that requires a political, social and media truce," he said.
The national dialogue is a key aspect of the blueprint drawn up by mediators and agreed to by Ennahda earlier this month.
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It will bring together the country's bitterly divided political factions and should lead within one month to the resignation of Tunisia's first freely elected government and the formation of a caretaker cabinet of technocrats.
The roadmap also envisages the adoption of a new constitution, electoral laws and a timetable for fresh elections.
But the opposition umbrella group, which managed to mobilise large anti-government gatherings in the weeks after Brahmi's assassination but has since seen its protests dwindle in size, hopes to muster a big crowd on Wednesday afternoon.
The League for the Protection of the Revolution, a controversial pro-government militia, has called for a rival demonstration in the morning, in the same central boulevard, raising the possibility of violence.
Within this context, Tunisian media on Tuesday voiced doubts about the talks bringing an end to months of political deadlock.
Daily Le Quotidien said the past few weeks had been marked by political "manoeuvres combining pressures, distractions and double speak, not to mention clashes and low punches."
Wednesday coincides with the second anniversary of the election of the National Constituent Assembly, just months after the uprising that toppled strongman Zine El Abidine Ben Ali and sparked the Arab Spring.
The assembly was tasked with adopting a new constitution within a year.
But the deadline has been repeatedly pushed back amid political wranglings between the Islamists, their coalition allies and the opposition, with the country lacking stable government institutions and a timetable for fresh elections, nearly three years after the revolution.
Tunisia has also been rocked by violence blamed on jihadist groups suppressed under Ben Ali.