The Yewwi Askan Wi coalition, whose name means "Liberate the People" in the local Wolof language, said it would ignore any bans on the demonstrations, which it called for Tuesday and Wednesday.
Sonko is expected in court Thursday after being sued by Tourism Minister Mame Mbaye Niang for "defamation, insult and forgery." He accuses Sonko of defaming him in relation to his management of a community agriculture program.
Posting on social media on Monday, the coalition announced a "giga meeting" for Tuesday afternoon followed by "national marches" throughout Senegal on Thursday.
"We will demonstrate with or without authorization," Sonko said at a press conference last Friday. "We are not calling for a coup d'etat or a civil war, but for the exercise of a constitutional right."
The demonstrations will denounce the "arbitrary arrests" of journalists and political activists and "the instrumentalization of the justice system against opponents," Sonko added.
On February 10, dozens of people were arrested following clashes, ransacking and looting in the city of Mbacke in central Senegal after a Sonko rally was banned.
The opposition accuses the government of systematically banning its demonstrations.
The government has denied any violation of political opponents' rights, saying the law is applied fairly in Senegal, a country with a reputation for stability in a region where political turbulence is widespread.
In addition to the defamation lawsuit, Sonko is facing separate charges of rape and making death threats, based on a complaint by an employee at a beauty salon where he was getting a massage.
On January 18, he was referred to a criminal court on those charges.
The two legal affairs could determine whether Sonko will be allowed to run in the presidential elections set for February 2024.
President Macky Sall, who was elected in 2012 and again in 2019, has not confirmed or denied whether he intends to override the constitution and seek a third presidential term next year.