Senegal's President Macky Sall has told the government to take measures to preserve public order after protests over the trial of opposition leader Ousmane Sonko left at least one person dead.

Violence has flared in several cities since Sonko's defamation trial opened on 16 March.

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

Sonko alleges that Niang has been criticised by an independent review body over the minister's handling of a fund intended to help young people find work in the farming sector. Niang denies that allegation.

The hearing of Sonko's case has been postponed until 30 March. If found guilty, the opposition leader could be declared ineligible to contest next year's presidential election. Sonko claims that the judiciary is being used to exclude him from the vote.

One dead in Casamance

One man was killed in the southern region of Casamance on Tuesday during clashes between Sonko supporters and police, according to local officials.

President Sall told the government to "take all appropriate measures to ensure the absolute security of goods and people", according to an official statement.

The president went on to say that "it was imperative to protect democracy and public order in Senegal," a country generally regarded as a bastion of stability in a troubled region.

Sonko, a fierce opponent of Sall, has been released from hospital after five days of medical treatment required after he was allegedly sprayed with "a toxic substance" and manhandled while authorities escorted him to a courthouse last Thursday.

The French AFP news agency reports the police on Wednesday arrested El Malick Ndiaye, the public relations official of Sonko's Senegalese Party of Africans United for Work, Integrity and Brotherhood.

Ndiaye is accused of "spreading false information and discrediting state institutions".