Senegal is holding a tightly-contested presidential run-off election between its 85-year-old president and a rival who has the backing of a dozen other opposition leaders.

Voters formed long lines in the capital, Dakar, on Sunday for a poll that analysts say could threaten the stability of one of Africa's most peaceful democracies.

President Abdoulaye Wade is running for a controversial third term against former prime minister Macky Sall.

Senegal's constitution limits the president to two-terms and Mr. Wade's bid for a third mandate has sparked deadly riots.

During voting on Sunday, police fired tear gas to disperse hundreds of Wade supporters at one polling station just before the president arrived to cast his ballot. Mr. Wade was booed by citizens during the first round of voting on February 25.

President Wade garnered 35 percent of the vote and Mr. Sall came in second with about 27 percent in the initial poll.

Since then, 12 other opposition leaders have backed Mr. Sall in his bid to unseat the incumbent.

President Wade has said he needs more time to finish projects, including construction of a new airport in Dakar. He also said he could run for a third term because constitutional term limits took effect after he was first elected. Critics have accused the president of focusing on infrastructure projects while ignoring the needs of the people.

Former prime minister Sall has promised to reform the government and to lower basic food costs.