A large number of Senegalese police have deployed in central Dakar ahead of a rally against President Abdoulaye Wade's re-election bid.
The opposition grouping, June 23 Movement (M23), says Mr Wade should be barred from seeking a third term.
Two people were shot dead on Monday in Podor town during protests after a court ruled Mr Wade's bid was legal.
The march had been banned but a BBC reporter in the city says it has now been authorised.
The M23 say they intend to march on the presidential palace.
Elections in the West African country - often held up as one of Africa's model democracies - are due on 26 February.
The BBC's Abdourahmane Dia in Dakar says many schools have sent their students home early.
He says some of the police who had deployed in the Place de l'Obelisque, where the rally is being held, have now moved away from the square.
Earlier, prominent activist Alioune Tine, a member of the M23 which was formed last year, was freed from police custody without charge after two days in detention.
After his release, he told the AFP news agency that police had asked him "many questions on the demonstrations" planned by M23.
Our correspondent says there are fears Tuesday's demonstrations could degenerate into violence.
But the opposition says it has a constitutional right to hold mass rallies across the country.
Our reporter says the bodies of two demonstrators killed in the northern town of Podor on Monday have been sent to Dakar for autopsies.
An eyewitness in Podor told Amnesty International that the police opened fire on an anti-Wade demonstration in the town.
"We were walking peacefully when suddenly security forces dressed in blue and belonging to the gendarmerie fired at the marchers with live bullets. People fell in front of me," the witness said.
The director of Amnesty International in Senegal called for restraint from both sides on Tuesday.
"We have to make sure their [protest organisers] followers are informed, that this right doesn't mean that they can attack security forces, throw stones, because it is this kind of behaviour that is the basis of police repression," Seydi Gassama told the BBC's World Today programme.
Mr Wade's campaign spokesman, Amadou Sall, said people had a right to protest, but not violently.
"Unfortunately two persons have died and we are very sorry and we apologise for this situation," he told the BBC's Focus on Africa programme.
"Every time the opposition protesters are in the street, each time there is a lot of violence and the police forces have not the elements to control this," he said.
If opposition supporters do not want Mr Wade as president they should go to the polls next month, Mr Sall said.
"Every five years the Senegalese citizens have the right to express themselves… on 26 February if they decide they don't want no more Abdoulaye Wade for president, Abdoulaye Wade will not be president, so it's all in their hands," he said.
"It is the same thing in all strong democracies and we consider that Senegal is a strong democracy."
Senegal's constitution has a two-term limit but the constitutional court has ruled this does not include Mr Wade's first term, which began before the clause was adopted.
At the same time as allowing Mr Wade to stand, the court ruled singer Youssou N'Dour and two other opposition candidates could not run.
The move has come in for international criticism.
"We are concerned that the decision by President Wade to seek a third term... could jeopardise the decades-long record that Senegal has built up on the continent for democracy, democratic development and political stability," Reuters news agency quotes US Deputy Secretary of State William Burns as saying.
France, the former colonial power, regretted that not all political viewpoints would be represented in the forthcoming elections, its foreign ministry said.
Opposition parties and activists have called for "national resistance" against Mr Wade's third-term bid.
Once a veteran opposition leader himself, Mr Wade was first elected in 2000 - ending 40 years of rule by the Socialist Party.