Leftist Belaid, 48, was a fierce critic of Islamist party Ennahdha, which was in power when he was gunned down, and his supporters have accused his political opponents over the killing.
The shooting was claimed by jihadists linked to the Islamic State group and the Tunisian authorities said the suspected hitman died during a 2014 operation.
Twenty four defendants have been on trial for alleged involvement in Belaid’s murder since 2015 but the case has been repeatedly postponed.
As relatives laid flowers on Tuesday at the site where he was shot dead they slammed the authorities for not fully probing the murder.
“I demand the truth, the whole truth,” his wife Basma Khalfaoui told AFP.
“I ask that all those who were behind it and carried it out, and all those who are hiding the truth… really be punished.”
Belaid’s death and the killing of lawmaker Mohammed Brahmi several months later rocked Tunisia as it looked to transition to democracy after the 2011 ouster of dictator Zine El Abidine Ben Ali.
The fallout from the murders saw Ennahdha forced to give up power in favour of a technocrat government just after the North African state adopted a new constitution.
Hamma Hammami, from the Popular Front coalition to which Belaid belonged, labelled his killing a “state crime” and said the murder trial had implicated those in power at the time of his death.
Belaid’s relatives say that Islamist Ennahdha at the very least tolerated a rise in radical sentiment at the time.
The party has denied all links to the killing and says it blacklisted hardline Salafist group Ansar al-Sharia after the murders.