Updated

Tens of thousands of Tunisians marched in support of their embattled government as opposition protesters renewed calls for the Islamist-led administration to step down, heightening a crisis sparked by a political assassination.

The ruling Ennahda party claimed that 200,000 people turned out for the mass rally in Kasbah square in the capital Tunis although police offered no turnout figures and AFP reporters on the scene estimated the number at tens of thousands.

The protest, by far the biggest yet staged in support of the government since the crisis erupted, attracted a wide range of people including several families, waving Tunisian and Ennahda flags but also symbols of the Salafist movement.

The crowd chanted slogans including "legitimacy", "the people want Ennahda" and "God is greatest" in support of the ruling party which organised the rally in response to growing calls for it to step aside.

Ennahda party leader Rashid Ghannouchi told the crowd: "Those who thought that the Egyptian scenario could be repeated here were all wrong."

"Tunisia was an inspiration with its revolution and it will not import a coup," said Ghannouchi, in reference to the Egyptian army's July 3 deposing of the country's president.

One demonstrator, Mohamed Hamrouni, a 25-year-old student, said: "The opposition has to stop calling the government into question, because it is the only way to complete our democratic transition."

Meanwhile, several thousand opposition demonstrators also staged what has become a nightly rally, witnesses told AFP.

The rallies passed off peacefully.

Tunisia has been gripped by a political crisis since the assassination in February of opposition politician Chokri Belaid.

The crisis was further stoked by the killing of MP Mohamed Brahmi, who was shot dead outside his home in a Tunis suburb on July 25.

A coalition of opposition parties called for a rally on Tuesday to demand the departure of the Islamist-led government and the dissolution of the National Constituent Assembly.

The parties chose the date for their planned march to fall exactly six months after Belaid's assassination.

Ennahda has rejected mounting calls from its detractors who are urging it to quit for failing to prevent the murders of Belaid and Brahmi, and for its inability to rein in radical Islamists.

At a news conference earlier Saturday, Prime Minister Ali Larayedh stressed the need for "national unity" and reiterated that his government would not step down.

His party, which emerged victorious in the October 2011 elections, has proposed to enlarge the coalition government.

Meanwhile, Tunisian troops continued their operations against Islamists in a remote mountain range after a deadly ambush on soldiers heightened the crisis.

The military launched ground and helicopter raids on Friday in the Mount Chaambi area near the border with Algeria where Islamist militants, including veterans of a revolt in northern Mali, are suspected to be hiding.

Eight soldiers were found in the area on Monday with their throats cut after being ambushed by militants.

Authorities have remained tight-lipped about the assault since claiming the army had the militants surrounded and calling on them to surrender or die.

Larayedh declined to comment on the operation at the news conference.

The government has acknowledged that the country faces a growing threat from terrorism.

But interior ministry spokesman Mohamed Ali Aroui insisted on Saturday that there was no "specific" threat of terrorist attack.

"The risk of an attack is the same in Tunisia as it is in France or anywhere else in the world," he told AFP.

Radical Islamists have grown in influence since the 2011 uprising that toppled long-standing president Zine El Abidine Ben Ali, and have been blamed for a wave of violence over the past two years.

Meanwhile, the interior ministry said a "religious extremist" was killed and another wounded in two separate incidents while handling explosives.

And police said a suspect package was found in Tunis containing a letter warning security forces to withdraw from Mount Chaambi.

Separately, authorities said late Saturday they had foiled another political assassination, arresting two "very dangerous terrorists" and seizing weapons.

Since Brahmi's death, around 60 politicians have pulled out of the National Constituent Assembly that is drawing up Tunisia's long-delayed new constitution.

One of them, Karima Souid, told AFP on Saturday that Tunisia needed a "national salvation government."

"The opposition does not want to govern but would like... the formation of a national salvation government," she said.

Tunisian authorities have pointed to links between the Chaambi militants, the assassins of Brahmi and Belaid and Tunisia's main Salafist organisation Ansar al-Sharia, which denies the accusation.