The planned protest comes amid political turmoil in nearby Egypt, where the army overthrew Islamist president Mohammed Mursi last month after millions of protesters took to the streets demanding his departure.
It was due to take place at 1700 GMT outside the national assembly in Tunis, where tens of thousands turned out for similar protests earlier this month and where an evening concert is planned by artists opposed to the ruling Ennahda party.
The opposition National Salvation Front is hoping the demonstration will trigger a week of protests across the country that will force Ennahda’s resignation, with the protests since Mohamed Brahmi’s murder concentrated in the capital.
Brahmi was the second opposition politician to be assassinated in six months. The attack plunged the country into a fresh political crisis, sparking opposition calls for the ouster of the Ennahda-led coalition and the formation of a non-political administration.