More than 40,000 people massed Tuesday outside Tunisia's National Constituent Assembly to demand the resignation of the government led by the moderate Islamic movement Ennahda.
The elected body had earlier suspended its work until the government and opposition open negotiations over a showdown sparked by the July 25 murder of opposition lawmaker Mohamed Brahmi.
Many of the protesters carried pictures of Brahmi as well as another opposition politician, Chokri Belaid, who was gunned down in February.
Both murders have been blamed on radical Islamists, with the Ennahda-led cabinet criticised for failing to rein in the extremists.
Assembly speaker Mustapha Ben Jaafar, meeting a key demand of the protesters, said earlier Tuesday that the body's work would be suspended "until the start of a dialogue, in the service of Tunisia".
Brahmi's killing has already prompted many opposition members to boycott the National Constituent Assembly (ANC).
The opposition refuses to hold talks with the government unless it steps down, while Ennahda rules out any dialogue conditional on its ouster.
In the face of terror threats facing Tunisia, Ben Jaafar condemned the failure of the political leadership to strike a compromise.
"Despite the gravity of the situation and instead of working towards unity, unfortunately party leaders have gone in the opposite direction, towards division, by mobilising" street protests, he said.
Since the ANC was elected in October 2011, political leaders have failed to find a consensus on a new constitution following that year's revolution that ousted longtime president Zine el Abidine Ben Ali.
The suspension of the assembly's work throws into question Prime Minister Ali Larayedh's target of the ANC adopting a new constitution and electoral law by October 23 ahead of a December 17 election.
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Ennahda chief Rached Ghannouchi, quoted in La Presse newspaper on Tuesday, said the government will not step down under pressure from the street, while Larayedh charged that demonstrators were hampering efforts by security forces to root out gunmen linked to Al-Qaeda.
"There are excessive demands at protests for the dissolution of the elected government," Ghannouchi told La Presse. "In democratic regimes, protests don't change governments; it's under dictatorial regimes that a demonstration is able to topple a regime."
A mixed bag of opposition parties, ranging from extreme left to centre-right, called for the late-evening protest necessitated by the Muslim fasting month of Ramadan, to mark six months since Belaid was gunned down outside his home.
Tunisia has been rocked by anti-government protests since the MP's killing, and protesters have clashed with police, who have used tear gas to disperse them.
The powerful, half-million strong Tunisian General Labour Union (UGTT), has issued a call for its members to join Tuesday night's demonstration.
The ANC earlier the same day gathered to discuss the "terrorist crisis" as security forces pressed on with a vast operation to hunt down militants holed up in the rugged Mount Chaambi region near the Algerian border.
Speaking to the assembly, the premier had harsh words for demonstrators, saying their activities meant security forces "are obliged to be in the streets when they should be participating in the battle against terrorism."
Defence Minister Rachid Sabbagh told the ANC that the armies of Tunisia and Algeria would "reinforce their cooperation, particularly to arrest the Chaambi terrorists."
"We will continue our operations until all the terrorists have been killed or arrested."
Ghannouchi also dismissed critics who say authorities are being soft on radical Islamists, who have grown in influence since the popular uprising that toppled Ben Ali two years ago. "There are 500 to 600 terrorist suspects imprisoned in jails," he said.
Larayedh has also ruled out the government's resignation, offering instead to broaden the coalition.