Zimbabwean President Robert Mugabe has confronted the toughest challenge to his 28-year rule at elections that the opposition said were rigged before polls opened. Voting was generally peaceful, with Zimbabweans standing in lines for hours, but African observers questioned thousands of names on the official roll. The 84-year-old Mugabe, in power nearly three decades, dismissed rigging charges. "I cannot sleep with a clear conscience if there is any cheating," he said Saturday after voting and promising to respect results. "If you lose an election and are rejected by the people, it is time to leave politics." Preliminary results are expected by Monday. If no candidate wins 50 percent plus one vote, there will be a runoff. An observer from the Pan-African Parliament said the longest lines in Harare were at two polling stations on the edge of a vacant plot where 8,450 people had registered as residents. The observer, who spoke on condition of anonymity because he was not authorized to speak to the media, gave an Associated Press reporter a letter to the Zimbabwe Electoral Commission asking for an explanation. Running against Mugabe are opposition leader Morgan Tsvangirai, 55, who narrowly lost disputed 2002 elections, and former ruling party loyalist and finance minister Simba Makoni, 58. Makoni threatens to take votes from both the opposition and the ruling party. The economic collapse of Zimbabwe has dominated the campaign. A nation that once fed itself and helped feed its neighbors now has a third of the population dependent on international food handouts and money sent from refugees in the diaspora. Unemployment is running at 80 percent - the same percentage surviving on less than $1 a day. Inflation is the highest in the world at more than 100,000 percent and people suffer crippling, sometimes lethal, shortages of food, water, electricity, fuel and medicine. Opposition leaders accuse Mugabe of dictatorship and destroying the economy. Mugabe calls his opponents stooges of former colonial ruler Britain. He says the nation must make sacrifices to overcome its colonial legacy. A parliamentary candidate for Mugabe's party in Bulawayo, Judith Mkwanda, reported two explosions outside her home that shattered windows just after midnight. Police said it was firebombed. No injuries were reported. Tsvangirayi's party said the home of one of its agents was set ablaze Saturday in northeast Zimbabwe. The independent Zimbabwe Election Support Network's monitors reported a heavy police presence at polling stations, ostensibly to help illiterate voters and allowed under a belated presidential decree that breaks an agreement signed with the opposition. The opposition said it was intimidation. Tendai Biti, a senior official in Tsvangirai's party, told reporters that his party's agents reported 200 voters - more than half of those who had cast votes in that polling place - were assisted by police in an area where the illiteracy rate was closer to 10 percent. Biti said opposition party agents had been barred from polling stations in several districts and that thousands of voters were turned away because their names were not on voters rolls or on flimsy excuses about identification particulars. His party also was investigating a report that six stuffed ballot boxes were found before voting got under way in one district, Biti said. Some 9,000 polling stations were set up for 5.9 million registered voters, but Biti said the real number was nearer 3.5 million because the rolls were inflated with names of dead or fictitious people and some of the 5 million Zimbabweans who have become economic and political refugees abroad. Zimbabweans had 12 hours to cast ballots, voting in a single day for the first time for president, 210 legislators, 60 senators and 1,600 local councilors. Zimbabwe barred several international media organizations from its elections as well as observers traveling from the US and EU. Zimbabwe's security chiefs, who have said they would not tolerate an opposition victory, said troops were on full alert to confront any violence. After independent monitors said 2002 elections were rigged, Western nations imposed visa bans and froze bank accounts for Mugabe and 100 of his cronies, but Mugabe has convinced many supporters that those limited sanctions are to blame for the country's woes. Zimbabwe's economy began unraveling when Mugabe ordered the often-violent eviction of white farmers to hand land over to blacks - mostly his relatives, friends and allies. Still, the land campaign won him support among blacks.