A student was killed in the northern town of Saint-Louis, according to associates of the man, as protests spread around the West African country.
Police fired tear gas to stop demonstrators getting to the Place de la Nation in central Dakar, where a rally had been planned. Hundreds of demonstrators threw stones at police and set fire to tyres.
Anger has mounted since President Macky Sall postponed a presidential election scheduled February 25 until December.
"The situation is deplorable. We came to pray and we got gassed. It's intolerable," Thierno Alassane Sall, one of the 20 candidates who had been due to vie for the presidency, told AFP.
Clashes spread to other areas of the capital, closing main roads, rail lines and main markets.
Demonstrations also took place in other towns, according to social network reports.
The death of the student in Saint-Louis was confirmed to AFP by a local hospital source speaking on condition of anonymity and an official at the university the student attended.
Police also dispersed a protest by about 200 people in Nioro du Rip, some 250 kilometers (150 miles) east of Dakar, an AFP reporter saw.
Authorities have not given a toll for the week of protests.
Unprecedented move
Sall said he postponed the election because of a dispute between parliament and the Constitutional Council over potential candidates who were not allowed to stand.
Sall said in July he would not stand again and has repeated that commitment several times. But opponents have accused him of a "constitutional coup" by delaying a new vote and keeping himself in power.
The postponement has been criticised by the United States and European Union. Senegal's parliament backed the move after security forces stormed the chamber and removed some opposition deputies.
The crisis has called into question the West African country's reputation for democratic stability in a region beset by military coups.
Protests usually require authorisation and rights advocates say authorities have routinely banned opposition demonstrations. Since 2021, dozens of people have been killed and hundreds arrested during unrest in the country.
"Senegalese must show their anger, and not just on social media," said the candidate Thierno Alassane Sall, who is no relation to the president.
Teachers were urged to walk out by education unions within the civil society platform Aar Sunu Election (Let's Protect Our Election).
At Blaise Diagne high school in Dakar, hundreds of pupils walked out of lessons with their teacher.
History and geography teacher Assane Sene said it was just the start of the battle. "If the government is stubborn, we will have to try different approaches."
At the Masjidounnour mosque in Dakar, 37-year-old Amadou Sy told AFP: "The message hasn't got through enough. But the situation in the country is deplorable, nobody's happy."
The vote by MPs to delay the election paves the way for President Sall -- whose second term expires in early April -- to remain in office until his successor is installed, probably in 2025.
A new date for the presidential election has been set for December 15.
The opposition has condemned Sall's move as a "constitutional coup."
On Friday, 14 opposition candidates lodged an appeal against the move with the Supreme Court.
But Sall is showing no signs of backing down, said Sidy Diop, deputy editor of Le Soleil daily. However, he added that the head of state is "in a very bad position."
If civil society and the opposition "manage to impose a balance of power unfavorable to the government and rally the international community, the president may then back down," he added.
International pressure may also have an impact, according to Alassane Beye, a lecturer-researcher at the University of Saint-Louis.
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