Tens of thousands of Tunisians crowded the streets of downtown Tunis on Tuesday to demand the transitional government's ouster, in the largest opposition protest since the country's political crisis began two weeks ago.

The secular opposition, angered by two assassinations in its ranks and emboldened by the army-backed toppling of Egypt's Islamist president, is trying to topple Tunisia's government led by the moderate Islamist party Ennahda.

It also wants to dissolve the Constituent Assembly, which is weeks away from finishing a draft constitution and election law.

In a surprise move that could tip the balance in the opposition's favour, the head of the Constituent Assembly suspended the body, saying it would not resume work until the government and its rivals held talks. Assemblyman and ruling party member Najib Mrad called the move an "unacceptable coup."

Tunisia is facing the worst political turmoil since autocratic ruler Zine El-Abidine Ben Ali was toppled. The crisis has been compounded by growing instability as Islamist militants step up their attacks.

"The people want the fall of the regime," shouted crowds crammed into Bardo Square, using the same slogan they popularised when Tunisians ousted Ben Ali in 2011 and sparked a wave of uprisings across the Arab world.

Tuesday's opposition protests mark the six-month anniversary of the assassination of leftist politician Chokri Belaid, one of the two opposition figures shot dead in recent months.

"This proves the desire for liberation from Brotherhood rule will not be broken," Belaid's widow, Basma Belaid, said, comparing Ennahda to Egypt's elected Muslim Brotherhood.

In June, the Egyptian army responded to mass opposition protests by deposing and detaining the president and launching a crackdown and arrest campaign against Brotherhood figures.

"This is a message to end their rule, from which we have only seen disasters such as violence and assassinations," Belaid's widow said.

RIFT EXPOSED WITHIN GOVERNMENT

Earlier on Tuesday, police shot dead an Islamist militant in a suburb of Tunis that houses several luxury hotels frequented by foreign tourists, an Interior Ministry official said.

Militant attacks have spiralled since the political crisis began. Last week, militants killed eight soldiers near the Algerian border in one of the deadliest attacks on Tunisian forces in decades.

The North African country has become increasingly polarised by rival street movements. Ennahda came out in a show of force a few days earlier, drawing the largest crowd since Ben Ali's ouster for a pro-government rally that it said topped 150,000.

But Assembly head Mustafa Ben Jaafar's decision to suspend the body ahead of Tuesday's protests could be a sign that the Ennahda-led ruling coalition is falling apart. Ben Jaafar's secular Ettakatol party was a junior member of the coalition government.

"(Ben Jaafar's) decision was a political one to the first degree that represents a huge rift in the governing alliance," said Tunisian political analyst Nourdine Mbarki. "This is a win for the opposition, which has been able to move the crisis inside the coalition government."

Ben Jaafar defended his decision as move to force a negotiated resolution: "This is in the service of Tunisia in order to guarantee its transition to democracy."

Organisers of the opposition protest on Tuesday said 100,000 joined their anti-government rally. A Reuters witness said that the numbers were close to Ennahda's rally, but still fell short.

Mongy Rahoui, a leader in the opposition's Popular Front, said of Tuesday's demonstrations: "These legions of crowds are a response to (Ennahda leader Rachid) Ghannouchi and we say to him we are ready for any referendum ... we are the ones with legitimacy on the streets. Your legitimacy is counterfeit."

On Monday, Ghannouchi told Reuters that removing the prime minister and dissolving the temporary Constituent Assembly were "red lines" he would not cross, but he was willing to hold a popular referendum or talks.

"We won't move until they (Ennahda) leave us alone," said protester Warda Habibi. "We were not afraid of Ben Ali and we're not afraid of Ghannouchi." - Reuters