By Clement Manirabarusha and Edmund Blair BUJUMBURA (Reuters) - A policeman and civilian were killed in clashes hours before the start of Burundi's presidential elections, already hit by opposition boycotts and protests over President Pierre Nkurunziza's decision to run for a third term. Opponents accuse Nkurunziza of violating the constitution by seeking another five years in office. Western donors and African states, worried about tensions in a region with a history of ethnic conflict, urged Burundi to postpone the poll.