Tunisia's Islamist-led government gathered in emergency meeting Monday as protesters demanding its ouster dug in outside parliament after another night of protests.
Tensions have spiralled in Tunisia since the murder on Thursday of opposition MP Mohamed Brahmi, the second anti-Islamist figure gunned down in six months.
Many Tunisians blame the government for the two killings, particularly for failing to rein in radical Islamists accused of a wave of attacks since strongman Zine El Abidine Ben Ali was toppled in a popular uprising in 2011.
The government led by Ennahda Islamist party was due to begin crucial talks at 0800 GMT.
In the afternoon, the powerful General Union of Tunisian Labour (UGTT) was to convene "to decide the fate" of the country, its secretary general Sami Tahri said.
Early on Monday, a group of 30 demonstrators protested outside the National Constituent Assembly (NCA) for a fourth day running, after night demonstrations by supporters and opponents of Ennahda.
Police used tear gas to disperse the protesters, an AFP correspondent reported.
Overnight around 10,000 people demonstrated for and against the government on Bardo Square outside the parliament building.
Police vans and metal barricades separated the two camps as numbers swelled after the iftar meal that breaks the dawn-to-dusk fast observed by the faithful during the Muslim holy month of Ramadan.
Opposition MPs and secular politicians joined protesters calling for the dissolution of the NCA and the resignation of the government, chanting: "The people want the fall of the assassins."
On the other side, government supporters chanted back: "The people are Muslim and will not capitulate".
"Those who boycott the ANC betray Tunisia," Ennahda MP Fathi Ayadi told AFP. "We will resist until the objectives of (2011) are achieved and there won't be a coup in this country."
He was alluding to the July 3 coup in Egypt which deposed elected Islamist president Mohamed Morsi. Since then the Arab world's most populous nation has been rocked by deadly violence.
Tunisia's rival protesters have called for more demonstrations outside the parliament building.
But NCA speaker Mustapha Ben Jaafar has called for "restraint" and urged deputies to return and resume work on a much-delayed constitution, one of the thorniest issues in post-revolution Tunisia.
Brahmi's murder has fuelled anger across Tunisia and dozens of MPs are boycotting parliament in protest at what many say is the government's failure to track down his killers.
On Monday, politician Hedi Ben Abbes, a member of the centre-left Congress for the Republic, one of Ennahda's coalition partners in the government, said he was quitting the party.
Authorities have blamed radical Salafists close to the Al-Qaeda-linked group Ansar al-Sharia of gunning down Brahmi, whose body was riddled with 14 bullets outside his home.
They say the same gun used to assassinate opposition politician Chokri Belaid in February killed Brahmi.
But Ansar al-Sharia denied any responsibility, calling Brahmi's murder "a political assassination" which "only profits remnants of the former regime and lackeys of the Zionists and Crusaders".
Hundreds of thousands of mourners thronged the streets of Tunis on Saturday for an emotionally charged funeral procession to El-Jellaz cemetery, where Brahmi was buried next to Belaid.
Slogans vowing to "avenge" Brahmi and Belaid rose from the sea of mourners.
Anti-government protesters then assembled outside the NCA and clashed with police who fired tear gas to disperse them and an opposition MP was injured by a blow to the head, AFP correspondents reported.
The demonstrators have kept up the momentum since then and have squared off with government supporters outside parliament.
Interior Minister Lotfi Ben Jeddou on Sunday pledged to guarantee the safety of anti-government demonstrators, leftist MP Samir Taieb said after meeting the minister.
Taieb, who has been calling on the government to step down and hand over to an administration of national unity, said nearly a third of the 217 members of the assembly were boycotting its sessions.