Senegal riot police have fired tear gas and sprayed protesters with water to break up a thousands-strong rally in Dakar demanding that President Abdoulaye Wade drop plans to seek a third term.
Private radio station RFM said a man was run over by the police's armoured-personnel carrier on Tuesday.
A reporter for the Associated Press news agency saw the young man fall to the ground after the carrier forced its way through the protesting crowd.
Police chief Harona Sy confirmed the death of the youth, but denied police responsibility. Sy said he had checked all of their vehicles and found no traces of blood, according to the state news agency.
"If there was the death of a man, then maybe we should talk about an accident, and we have opened an investigation,'' Sy said.
"I spoke to an older woman and asked her why she was there. She said that Wade needed to leave. She said she had a large family, and when I asked her if she was there for them she said: 'I am here for me, I am here for everyone!' "
- Megan Radford, journalist in Daka
The AFP news agency reported that angry youth had taunted security forces, throwing stones at police who stood some 300 metres away from the square where the demonstration was being held. They burnt tyres and engaged in running battles with police in side streets after the square emptied out.
The demonstration, organised under the umbrella activist organisation M23 comes just a day after two civilians were killed by paramilitary police in a similar demonstration in the northern town of Podor.
Thousands of people gathered, held up signs and chanted slogans in protest against the decision taken by the country's top legal body to allow President Wade's candidacy in next month's election.
Wade is seeking a third term, even though the constitution was changed soon after he took office in order to impose a two-term limit.
Megan Radford, a journalist based in Dakar, told Al Jazeera the demonstrations took some time to begin as crowds trickled into the square into the late afternoon and early evening.
"People only started arriving late but within the next few hours, the number of protesters had increased dramatically. I spoke to an older woman and asked her why she was there. She said that Wade needed to leave. She said she had a large family, and when I asked her if she was there for them she said: 'I am here for me, I am here for everyone!'"
Earlier on Tuesday, Mor Ngom of the M23 movement said that authorities had "accepted" their application to hold a mass rally. The statement came after the interior ministry had earlier said that that the opposition had not gained authorisation to hold a rally.
"Today's bloodshed marks a dramatic escalation in the violence that has plagued Senegal in the run-up to its elections."
- Salvatore Sagues, Amnesty International
Serigne Mbacke Ndiaye, Senegal's presidential spokesman, said: "The real combat is the one we must lead to hold a transparent election... being a candidate means nothing."
"We deplore the will (of the opposition) to lead the country into chaos... We don't want Senegal to go up in flames."
The constitutional council on Monday dismissed all appeals against Wade's candidacy, leaving no legal recourse for opponents who accuse him of carrying out a constitutional coup.The opposition has vowed to continue mass resistance to force Wade to step aside prior to the February 26 presidential vote.
Rights group Amnesty International urged the government to halt a clampdown on protesters following the reported deaths in Podor.
"Today's bloodshed marks a dramatic escalation in the violence that has plagued Senegal in the run-up to its elections," Salvatore Sagues, the UK-based rights body's West Africa researcher, said.
'Too late' to stop Wade
The US urged 85-year-old Wade to allow power to pass "to the next generation".
"While we respect the process, the political and legal process in Senegal, the fact that he's now been cleared to run, our message to him remains the same: that the statesmanly-like thing to do would be to cede to the next generation, and we think that would be better," Victoria Nuland, a state department spokeswoman, said.
"Our view is that Senegalese democracy is strong enough to move to the next generation."
Senegal, which some see as a beacon of democracy among troubled neighbours, is the only country in West Africa not to have had a coup since the end of the colonial era.
El Hadj Amadou Salla, minister of state and a senior Wade campaign official, said it was "too late" to prevent Wade running since his candidacy had already been validated.
The opposition argues that the constitution allows a president to serve only two consecutive terms, but Wade says the law, which was amended in 2008, does not apply retroactively and cannot take into account his previous two terms.
Wade has dismissed opposition protests as "temper tantrums".
In a recent interview with a local news website, Wade said he needed three more years to complete his projects, fuelling speculation that he wanted to line up a successor.