Liberian incumbent Ellen Johnson Sirleaf on Saturday accused her main challenger in presidential elections of \"violating the constitution\" by calling for a boycott of a run-off vote next week. \"Mr Winston Tubman has called on Liberians to give up their franchise, their right to vote. He has called people to violate the constitution. When you start violating the constitution, where do you stop?\" Sirleaf said in a statement at the presidency headquarters.