The protesters set off flares and chanted slogans
Thousands of Tunisians have rallied in front of the National Assembly in the capital Tunis calling for the Islamist-led government to resign.
The opposition National Salvation Front has called for a week of protests over what it says is the government's inability to guarantee security.
The protests come a month after the assassination of a prominent opposition politician.
It was the second such politically-motivated killing this year.
The governing Ennahda party has offered to support an all-party government but has ruled out calls to dissolve the constituent assembly or remove Prime Minister Ali Laaraiedh.
"The people want the fall of the regime," chanted the crowds, repeating the slogan used when Tunisians ousted President Zine el-Abidine Ben Ali in 2011. His downfall triggered revolts across the Arab world.
Correspondents say Tunisian opposition parties have recently been emboldened by the Egyptian army's ousting of Islamist President Mohamed Morsi.
Witnesses said police monitored Saturday's protests but there were no reports of violence.
'Living in fear'
Protester Nejet Brissi, 41, said she wanted the government to step down and make way for a caretaker administration to oversee fresh elections.
"Since Ennahda came to power we have been suffering," she said.
"We have been crushed by the rising cost of living. There is no security any more. We are living in fear of terrorists."
Opposition politician Mohamed Brahmi was shot on 25 July, almost six months after secular opposition leader Chokri Belaid, a fellow leftist politician, was killed.
Tunisia's powerful UGTT trade union has been mediating between the Islamists and the opposition.
Ennahda said on Thursday it accepted in principle a proposal to form a technocratic government, but only after further negotiations.
The UGTT plans to continue its mediation work next week.
"We hope that we will find a solution responding to the interests of the nation above all, and which satisfies the different parties," said UGTT secretary general Hocine Abassi, after holding talks with President Moncef Marzouki.