Tens of thousands of Tunisians gathered in a central square in the capital Tunis to back the ruling Islamist Ennahda party.
The party organised the rally in Kasbah square in support of what it termed its "legitimacy to govern".
Government opponents were planning to hold a counter protest, calling for Ennahda to quit after the second murder of an opposition figure this year.
PM Ali Larayedh earlier appealed for calm but said he would not step down.
The government has faced street protests after the 25 July killing of left-wing MP Mohamed Brahmi, the leader of the small left-wing Popular Movement party. In February, prominent secular opposition leader Chokri Belaid was also assassinated.
In addition to the social unrest, Tunisia has also been fighting a rise in religious extremism.
Armed forces launched an offensive against Islamist militants near the Algerian border on Friday after suspected al-Qaeda gunmen ambushed and killed eight soldiers.
Protesters gathered in Kasbah square have been chanting "We defend the legitimacy of the government" and "Ennahda will not back down," the BBC's Ahmed Maher reports from Tunis.
There is heavy police presence in the square and security is tight, our correspondent adds.
Meanwhile opposition groups were said to be preparing for a counter-rally outside the Constituent Assembly in the capital.
"Tunisia is in need of national unity," Mr Larayedh said earlier.
"I call for calm so that the army and security forces can combat terrorism and not waste its efforts on protests."
The prime minister held talks on Saturday with various political groups to address the political and security crises.
Earlier this week, Education Minister Salem Labyedh was forced to resign over the killing of Mr Brahmi. The government blamed Salafist hardliners for the murder.
Tunisia is the birthplace of the Arab Spring, but it has been in turmoil since the overthrow of long-serving ruler President Zine al-Abidine Ben Ali in 2011.
The secular Ettakatol party has called for the Ennadah-led coalition to step down because of the tensions following the killing of Mohamed Brahmi,
But Mr Larayedh has vowed the government will fulfil its mandate and hold elections in December.