Tunisia's ruling Islamists and opposition were to begin hard-won negotiations on Wednesday to end months of political deadlock, with Prime Minister Ali Larayedh expected to announce his government's resignation.
Ahead of the long-awaited national dialogue, which mediators hope will mark a crucial step in the country's democratic transition, rival protests are planned in the capital's central Habib Bourguiba Avenue, epicentre of the January 2011 revolution.
A coalition of secular opposition parties has called a demonstration early afternoon to demand the immediate departure of the government led by ruling Islamist party Ennahda, which it accuses of clinging to power.
Separately, the League for the Protection of the Revolution, a controversial pro-government militia, has urged supporters of Tunisia's first freely-elected government to defend its "legitimacy," raising fears of violence.
Police from dawn deployed along the central Tunis boulevard and partially blocked traffic.
Before the national dialogue begins, Larayedh is due to convene an extraordinary cabinet meeting and will make a statement afterwards, at around 1330 GMT, his office said.
Parliament speaker Mustapha Ben Jaafar said he expected the prime minister to announce his commitment to resign, allowing negotiations between Tunisia's bitterly divided factions to end the political paralysis gripping the country since the July killing of opposition MP Mohamed Brahmi.
"In principle, the government will announce its commitment to respecting the roadmap and its resignation within a few weeks," he said in a televised interview on Tuesday evening.
Larayedh has previously stated that he would step down only once a new constitution has been adopted, in line with the roadmap drawn up by mediators and agreed to earlier this month by his ruling Islamist party Ennahda.
Some 60 opposition MPs who have been boycotting parliament since the political crisis erupted, also said they had received assurances the national dialogue would begin with the government announcing its resignation.
According to the political roadmap, the talks will lead within three weeks to the formation of a new caretaker cabinet of technocrats.
Negotiators will also have one month to adopt a new constitution, electoral laws and a timetable for fresh elections, key milestones in the democratic transition which has effectively been blocked by wrangling between the Islamists, their coalition allies and the opposition.
Wednesday's planned dialogue comes exactly two years after the election of the National Constituent Assembly, which followed the uprising that toppled veteran strongman Zine El Abidine Ben Ali and brought Ennahda to power.
The Islamists were heavily repressed under the Ben Ali regime.
Since triumphing in the parliamentary elections in October 2011, they have been weakened by accusations that they have failed to fix Tunisia's lacklustre economy and prevent attacks by Islamist militants.
After three months of political uncertainty, unkept promises and a false start to the national dialogue on October 5, the Tunisian press has grown increasingly critical of the ruling elite and sceptical of efforts to end the crisis.
"A national dialogue starts on the day of a symbolic anniversary. But it has been compromised by the prevalence of suspicion, deceitful language and ambiguity," Le Temps said in an editorial on Wednesday.
For Le Quotidien, another daily, "The moment of truth is approaching with giant steps" for Tunisia, which risks turning towards "a future still more uncertain and chaotic."
The opposition has repeatedly criticised the Ennahda-led coalition government for failing to stem a rise in jihadist violence that Tunisia has witnessed since Ben Ali's overthrow, with extremists blamed for the murder of Brahmi and another opposition MP in February.
The Islamists reject the accusations, pointing to a massive military campaign launched against jihadist groups in recent months, which saw nine suspected militants killed west of the capital last week in response to an attack which left two policemen dead.