Police opened fire with tear gas Saturday on a commercial avenue in Senegal's capital, as security forces wrapped up early voting in a contentious presidential race.

It marks the fourth straight day of protests before next week's critical election, which is pitting the country's 85-year-old president against a young opposition demanding his departure. The increasingly tense atmosphere on the ground has many concerned there may be unrest if President Abdoulaye Wade is declared the winner of the vote.

On Saturday morning, the country's 23,000-strong security force including police and military, lined up to vote early. European Union observers said the ballots had arrived on time and voting proceeded calmly.

The normally bustling capital of Senegal has been disrupted by protests every few days since January when the country's highest court ruled that Wade could be a candidate in this month's election. Just months shy of his 86th birthday, Wade is seeking another seven-year term in a nation where most people don't live past the age of 60.

If he wins, it would be his third term, even though he oversaw a revision to the constitution in 2001 imposing a two-term limit.

Shopkeepers pulled down metal grills to protect their businesses as the blast of tear gas being fired resonated in the capital's downtown core. The vendors that hawk their wares on the street hurried to pack up their goods.

Four people have been killed since the protests began, a death toll that is small compared to the election violence that recently gripped Ivory Coast, Guinea, Nigeria and Congo, but high for Senegal which has long been seen as a model of tolerance and stability.

Late Friday, police attempting to disperse a crowd launched a tear gas grenade inside a mosque belonging to the Tidiane Muslim "confrerie," or brotherhood. Enraged worshippers ran outside.

Senegalese TV showed an officer who braved the cloud of gas in order to kick away the spewing canister from the door of the mosque, in order to calm the anger.

The country's Muslim brotherhoods hold enormous sway, and on Saturday, Interior Minister Ousmane Ngom traveled to Tivaouane, home of the Tidiane's khalife, religious leader. A mob of angry youths encircled the house of the khalife, as Ngom was inside, according to Ngom's spokesman Mbaye Thiam who was reached by telephone in Dakar.

The state-owned news service said that the youths were preventing Ngom from leaving and that they had also set fire to a government building nearby.

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Sadibou Marone contributed to this report.