Voters in Senegal have begun casting their ballots on Sunday after an election campaign marred by violence. There have also been calls for incumbent President Abdoulaye Wade to forgo re-election to a third term. Senegalese began casting their ballots on Sunday after an election campaign marred by violence that saw calls for incumbent President Abdoulaye Wade to forgo re-election to a third term. Sporadic demonstrations began in Senegal late last month when the country\'s constitutional court ruled that 85-year-old Wade could run for a third term in office on February 26. Senegal\'s 2001 constitution states that a president can only serve two terms in office, but Wade argued that his first stint - from 2000 to 2007 - fell under the terms of the previous constitution, which imposed no such limits. Foreign diplomats issued last minute appeals for calm and a transparent vote, but a top African mediator failed to secure an agreement between Wade and his rivals over a shortened term for the incumbent if he won. \"We are watching the development of the situation closely. I have been concerned about what is happening there,\" United Nations Secretary General Ban Ki-moon told journalists on Saturday during a visit to Zambia. \"I sincerely hope that this election will be held peacefully in a credible, open and transparent manner so that the will of the people will be fully respected,\" he said. Former Nigerian President Olusegun Obasanjo, acting as a top envoy for the African Union, attempted to persuade Wade to give up power after two years if he is re-elected. \"We believe that this country must be prevented from chaos, from tragedy, from disaster,\" Obasanjo told journalists in Dakar. Thirteen candidates are registered for the election, with three former prime ministers among them. None has emerged as a clear favorite to grab a majority in the polls.