The army in Tunisia has blocked off a square where rival protesters confronted each other in Tunis on Monday.
Tensions have been growing over efforts to oust the government following last week's assassination of an opposition politician, the second such killing in six months.
Protesters in the southern city of Sidi Bouzid also clashed with supporters of the Ennahda party, which leads the transitional government.
The city was the cradle of the revolt that overthrew President Zine al-Abidine Ben Ali in 2011.
The secular opposition is now rejecting all concessions and reconciliation efforts by the government.
The transitional Constituent Assembly is due to complete a draft of a new constitution.
Seventy members of the transitional Constituent Assembly have left it and set up a sit-in outside the Assembly offices in Bardo square.
They demand that the Assembly, which has 217 members, be dissolved and elections called.
Cabinet ministers held an emergency meeting on Monday to discuss the crisis.
Afterwards prime minister Ali Larayedh said the government will not step down. The BBC reports he said it would fulfil its mandate and hold elections in December.