Tunisia's Islamist ruling party chief has said the government will not step down under pressure from the opposition, even as fresh protests were being readied Tuesday to demand just that.

Rached Ghannouchi's remarks, published by La Presse newspaper, will likely deepen a crisis that has gripped the North African nation since a political assassination in February was followed by another last month.

"There are excessive demands at protests for the dissolution of the elected government," Ghannouchi told La Presse.

"In democratic regimes, protests don't change governments; it's under dictatorial regimes that a demonstration is able to topple a regime.

"Unfortunately every time a tragedy hits us, we immediately call for the dissolution of the government and parliament," Ghannouchi added.

A mixed bag of opposition parties, running from the extreme left to the centre-right, is planning a massive protest in Tunis at 2000 GMT to mark six months since opposition politician Chokri Belaid was gunned down outside his home.

Government detractors say the Ennahda-led cabinet has failed to rein in radical Islamists blamed for Belaid's murder and the assassination of MP Mohamed Brahmi on July 25.

They are also calling for the dissolution of the National Constituent Assembly, which was elected nearly two years ago and has still not managed to complete its task of writing a new constitution.

The NCA was due to gather on Tuesday to discuss the crisis.

The powerful, half-million strong Tunisian General Labour Union (UGTT), has issued a call for its members to join Tuesday night's demonstration. However, it does not back demands for the NCA to be dissolved.

Meanwhile, the employers' federation, the Tunisian Union of Industry, Trade and Crafts, again urged on Tuesday the formation of a government of technocrats.

In the La Presse article, the Ennahda leader also dismissed critics who say authorities are being too soft on radical Islamists, who have grown in influence since the 2011 popular uprising that toppled strongman Zine El Abidine Ben Ali.

"There are 500 to 600 terrorist suspects imprisoned in jails," Ghannouchi said, without giving further details.

Tunisia has been rocked by anti-government protests since Brahmi's murder, and protesters have clashed with police who used tear gas to disperse them.

Authorities said Brahmi was murdered with the same gun used to kill Belaid.

On Sunday Prime Minister Ali Larayedh announced the first arrest of a suspect in the Belaid case.

Larayedh has also ruled out the government's resignation, offering instead to broaden the coalition and calling for a general election in December.

In an interview published by Belgian daily Le Soir on Monday, Ghannouchi also called for a referendum to give voters the chance to have their say in the standoff between the cabinet and protesters.

"Tunisians would pronounce on whether they support or oppose the process under way," he said.

As the political crisis festers, security forces are pressing their hunt for militants linked to Al-Qaeda holed up in the rugged Mount Chaambi region near the Algerian border.

The military has been hunting the militants since December, and stepped up its offensive after eight soldiers were killed in an ambush on July 29.