Why are people taking to the streets in Senegal?

Today is Independence Day in Senegal, but not everybody is celebrating. The day is being snubbed by the Yewwi Askan Wi opposition coalition. Last week, opposition supporters staged protests ahead of the trial of opposition leader Ousmane Sonko. On Sunday, he was found guilty of libel against Tourism Minister Mame Mbaye Niang, who he had accused of embezzlement. Sonko was given a two year suspended jail term. But he has accused the country's president of leading a "strategy of terror" to drive out political opponents. Shortly after this episode was recorded, President Macky Sall told the nation he is open to dialogue in a televised speech ahead of the independence celebrations. So, what’s behind the political unrest in Senegal?