El Hadji Malick Ndiaye, head of communications for Sonko’s party, was charged and electronically tagged over a message posted on Facebook on March 16 about the conditions under which Sonko was taken to court in Dakar on that day, under heavy police escort.
Sonko faces trial in a defamation case that could make him ineligible to contest next year’s presidential election.
The security forces had forcibly removed Sonko from his car and taken him to the court where he appeared, causing clashes in Dakar.
Those accompanying Mr Sonko accused the security forces of spraying them with a product to get them out of the car.
Ndiaye was arrested last Wednesday. On Monday he was charged with “disseminating fake news likely to discredit institutions and manoeuvres and acts likely to compromise public security or cause serious political unrest,” Moussa Sarr, one of his lawyers, told AFP.
Senegal, a rare island of stability in the troubled West African region, is entering another tense week with the resumption of the libel trial against Sonko on Thursday, and calls from the opposition for demonstrations on Wednesday and Thursday, as well as on April 3.
The opposition is demanding “the release as soon as possible of all political detainees” and calling for support for Sonko.
Senegal’s President Macky Sall has asked the government 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.
The presidential camp accuses Sonko of drumming up anger on the streets in a bid to escape justice.