Senegalese opposition leader Ousmane Sonko alleged on Tuesday he was being "illegally held" by security forces and urged the public to protest.
"I urge all Senegalese people to come out onto the streets," he said on social media, escalating a political tug-of-war with President Macky Sall's government.
Sonko, a critic of the government who came third in the 2019 presidential election, was declared "unreachable" by his party on Sunday after police in southern Senegal had forcibly taken him to the capital Dakar.
On Friday he had set out for Dakar in a convoy ahead of the outcome of a rape trial that could render him ineligible to contest next year's presidential election.
He denies the charges against him but failed to attend the first two hearings in the case, citing what he called fears for his safety.
The verdict is expected on Thursday, triggering fears of violence.
Senegal is traditionally a beacon of stability in troubled West Africa, but in recent years has been buffeted by turbulence that has at times turned deadly.
Sonko, 48-year-old president of the PASTEF-Patriots party, has a significant following among Senegal's huge population of young people.
When he was arrested in 2021, several days of clashes left at least 12 people dead.
His 500-kilometre (300-mile) "caravan of freedom" at the weekend drew crowds of supporters and led to clashes with security forces in which one man died.
He documented the journey live on social media for the first two days.
But there were no posts from Sunday onwards, when police took him to Dakar and blocked access to his house in the capital.
He resurfaced on social media in the early hours of Tuesday to say he was being "illegally held."
"How can a country that calls itself a democracy justify confining without any legal basis a citizen whose only wrong was to be a fervent and unflinching opponent of Macky Sall's regime?" he posted.
"Everyone should rise up as one because power should remain with the people."
He called for a "national resistance movement" to defend democracy and freedoms.
- Legal storm -
Feelings among Sonko supporters are running high ahead of the climax of the trial.
Young followers clashed with police in several areas of Dakar on Monday.
The trial is based on allegations of sexual assault and death threats, filed by a worker at a Dakar beauty salon.
Sonko has said he went there for a massage for chronic back pain and says the case against him is politically motivated.
His bid for the presidency has also been clouded by a six-month suspended sentence, handed down in March, for defaming Tourism Minister Mame Mbaye Niangtou.
Political tensions have also been stoked by Sall's refusal to rule out running for a third term as president, a move his opponents say would be unconstitutional.
Sall was elected in 2012, when the presidential term was seven years, and re-elected in 2019, when the term was reduced to five years.