Tens of thousands of Tunisians have crowded the streets of downtown Tunis to demand the government's ouster, in the largest opposition protests to hit the capital since the country's political crisis began two weeks ago.

The protest on Tuesday marks the six-month anniversary of the assassination of leftist politician Chokri Belaid, one of two opposition figures to be shot dead in recent months.

It comes hours after the embattled Constituent Assembly suspended its work indefinitely. Mustafa Ben Jaffar, the head of the assembly and secretary-general of the centre-left party Ettakatol, announced the suspension.

"I assume my responsibility as president of the ANC [assembly] and suspend its work until the start of a dialogue, in the service of Tunisia," he said on state television.

He was referring to a crisis sparked by another assassination of an opposition figure, which has already prompted many opposition members to boycott the assembly's sessions.

The assembly was only weeks away from finishing a draft constitution and electoral law that would move the country closer to new elections.

The country's secular opposition is trying to oust the Islamist Ennahda-led government and dissolve the transitional Assembly.

Protests have been held daily since the killing of leftist politician and assembly member Mohamed Brahmi on July 25, nearly six months after another leftist figure was gunned down.

More than 70 members of the assembly withdrew two weeks ago in protest at the two killings and organised a sit-in outside the assembly headquarters.

The assembly met on Tuesday morning despite the absence of protesting lawmakers.

Tunisians are facing the worst political crisis since the toppling of autocratic ruler Zine el-Abidine Ben Ali in 2011, in a revolt that sparked uprisings across the Arab world.