"The people have to mobilise to support Ousmane Sonko in this fight," Khalifa Sall, one of the coalition chiefs, told a press conference a day after Sonko was handed a six-month suspended term that could jeopardise his bid for the 2024 elections.
Sall joined other heads of the Yewwi Askan Wi alliance in urging followers to turn out in force for an already-scheduled rally due this Friday, and for another protest a week later, on May 19.
In March, Ousmane Sonko received a two-month suspended prison sentence in the libel case involving the tourism minister, a ruling that still allowed him to compete for the presidency in the February 2024 election.
But Monday's appeal hearing extended the suspended sentence to six months, said lawyer Boubacar Cissé, who represents the minister.
The ruling against Sonko is the latest twist in a legal saga that he has denounced as politically motivated - a standoff with the authorities that has fueled unrest in Senegal, widely viewed as one of West Africa's most stable democracies.
If Sonko does not successfully challenge the new sentence, it will likely preclude him from the race under the electoral code on candidates' eligibility, said Moussa Diaw, senior lecturer in political science at Gaston Berger University in the Senegalese city of Saint Louis.
"If this sentence is final, there is a good chance that his candidacy will be inadmissible," Diaw said. "This could seriously sully the political dialogue."
Sonko's lawyers declined immediate comment. He has six days to file an appeal with the Supreme Court.
The earlier lighter sentence had been interpreted as an effort by authorities to defuse tensions with Sonko's supporters, who have repeatedly taken to the streets to protest. Their anger has added to rising discontent in some quarters with President Macky Sall's failure to rule out running for a third term in office.
Senegal's constitution only allows two terms, but some fear Sall will use a recent tweak to the constitution to reset his mandate, which ends in 2024, allowing him to run again.
Sonko, who came third in the 2019 election, has clout among Senegal's youth, many of whom struggle with under-employment and poor economic prospects. After Monday's ruling, some students at Dakar's Cheikh Anta Diop University threw rocks at the dozens of riot police gathered outside the campus, who fired tear gas in response.