A Tunisian court on Wednesday sentenced four people to death and two people to life in prison on charges of participating in the murder of prominent political leader Chokri Belaid 11 years ago, the country's first political assassination in decades.

Belaid, a leftist politician, had been a fierce critic of the Islamist Ennahda party, claiming it turned a blind eye to violence perpetrated by extremists against secularists. He was shot dead in his car by gunmen on February 6, 2013.

Belaid's assassination shocked the country and sparked massive and violent protests at an already turbulent time shortly after the Arab Spring in 2011, when protests erupted first in Tunisia and then across the region toppling several longtime autocrats.

Near Tunis court, dozens of Belaid supporters gathered since Tuesday night, raising slogans demanding justice.

They chanted "Chokri is always alive" and "We are loyal to the blood of the martyrs".

While Belaid had only a modest political following, his criticism of Ennahda policies spoke for many Tunisians who fear religious radicals are bent on snuffing out freedoms won in the first of the revolts that rippled through the Arab world.

Months after Belaid’s assassination, Mohamed Brahmi another leftist was shot dead by gunmen also, and mass protests and political pressure forced the Islamist-led government to step down at the time.

Those involved in the assassination of Belaid and Brahmi belonged to the militia Ansar al-Sharia, an organisation classified as a "terrorist group" by the government in August 2013.

Belaid's family and secularist politicians accused Ennahda Islamist party's leaders of being behind the assassination when they were leading the government.

Ennahda deny strongly any connection to the assassination.

"The details concluded by the judicial circles clearly show evidence of the innocence of the Ennahda", Ennahda said on Wednesday in a statement.

It added that this verdict should restore respect to those who have been affected by false political accusations, especially the leader of Ennahda Rached Ghannouchi.

Ennahda called for opening a new page of major reconciliations and ending strife, exclusion and hatred.