Observers watch as a filled ballot box is sealed Saturday at a polling station in Harare. President Robert Mugabe faces the toughest challenge to his 28-year rule and the opposition is urging its supporters to defend their votes against an alleged ballot-rigging plot.

Harare, Zimbabwe ? Zimbabweans voted Saturday on whether to keep the ruler blamed by opponents for their country’s economic collapse, though President Robert Mugabe’s challengers claimed the election was rigged even before the polls opened.

The main opposition party said it was investigating reports of thousands of voters being turned away from polls and the discovery of stuffed ballot boxes in one district. African observers also questioned thousands of names on official rolls.

The election presented Mugabe with the toughest political challenge to his 28-year rule. He dismissed allegations that the vote was rigged to keep him in power.

“I cannot sleep with a clear conscience if there is any cheating,” Mugabe, 84, said after voting and promising to respect results. “If you lose an election and are rejected by the people, it is time to leave politics.”

Voting was generally peaceful, with Zimbabweans standing in lines for hours. Preliminary results are expected by Monday. If no candidate wins 50 percent plus one vote, there will be a runoff.

Running against Mugabe are opposition leader Morgan Tsvangirai, 55, who narrowly lost the disputed 2002 elections, and former ruling party loyalist and finance minister Simba Makoni, 58.

Makoni’s defection is a sign of growing dissent within Mugabe’s ruling Zimbabwe African National Union party. But while he could take support from Mugabe, Makoni also could divide the opposition vote.

Opposition leaders accuse Mugabe of dictatorship and destroying the economy. Mugabe calls his opponents stooges of former colonial ruler Britain and says the nation must make sacrifices to overcome its colonial legacy.

The economic collapse of Zimbabwe has dominated the campaign. The nation once fed itself and helped feed its neighbors, but now a third of its population depends on international food handouts and remittances from relatives abroad.

Unemployment stands at 80 percent – the same percentage that survives on less than $1 a day. Inflation is the highest in the world at more than 100,000 percent and people suffer crippling shortages of food, water, electricity, fuel and medicine.