Yet the battle around the political future of Mr. Sonko, 48, whose fiery rhetoric has made him popular among young Senegalese, has become the president’s biggest challenge. In the coming months, it could lead to the most serious test faced by Senegalese democracy in more than a decade, analysts say.

“Senegal finds itself in a thick fog, with lots of uncertainties,” said Alioune Tine, a rights expert and founder of the AfrikaJom Center, a Dakar-based research organization. “It has turned into a police state and, increasingly, an authoritarian one.”

There is no public proof that Mr. Sonko’s case has been politically motivated, but some academics, human rights observers and most opponents of Mr. Sall have raised questions about the lack of concrete evidence and the harsh treatment of Mr. Sonko throughout the proceedings. They have also in recent years warned of a steady erosion of democratic norms as several political opponents have been jailed and journalists arrested.

In recent months, police officers have been posted at traffic circles in Dakar; temporary bans on motorcycles to prevent quick gatherings of protesters have become a fixture in the capital; and demonstrators have faced a heavy-handed response from security forces, with clashes at times turning deadly. Protesters have also targeted the police, attacked gas stations and, this week, burned the house of Mr. Sall’s chief of staff.