Violence has flared in several cities since Ousmane Sonko's defamation trial opened on March 16, a case that could make him ineligible to contest next year's presidential elections.

The Yewwi Askan Wi coalition, whose name means "Liberate the People" in the local Wolof language, called for protests in Dakar next Wednesday and throughout the country on April 3.

The opposition is demanding "the release as soon as possible of all political detainees" and calling for support for Sonko, whose trial continues.

Senegal's President Macky Sall asked the government on Wednesday to take measures to "preserve public order" after the protests over the trial left at least one person dead.

A conviction for Sonko in this, or a separate rape case that has not gone to court, could prevent him from contesting next year's presidential election.

Sall has denied Sonko's accusation that the judiciary is being used to sideline him from the vote.

Protests and clashes have occurred in several towns. Many activists of Sonko's party have been arrested and some jailed.

Sonko is being tried for allegedly defaming Tourism Minister Mame Mbaye Niang, a member of President Sall's party.

The opposition leader has been receiving medical treatment since he was allegedly sprayed with tear gas and manhandled last week as authorities escorted him to a courthouse.

The next hearing in Sonko's trial has been postponed until March 30.