Sadeq al-Mahdi (C), leader of the opposition Umma party and candidate in the upcoming presidential elections, arrives to attend a meeting involving opposition parties in Khartoum April 1, 2010. Sudan's main opposition parties have withdrawn from this month's presidential election, a senior member of one of the groups said on Thursday, a decision that could wreck the vote and damage the faltering peace process. REUTERS/Zohra Bensemra (SUDAN - Tags: POLITICS ELECTIONS)