The government has barred Western observers like those from the European Union, but the African Union and the regional trade bloc, the Southern African Development Community, as well as local organizations, have been accredited in large numbers and will be watching at the polls.

And while most foreign journalists were barred during the last election, social media will be a prominent part of the campaign. Both Mr. Mugabe’s party, ZANU-PF, and its main challenger, the Movement for Democratic Change, are using Twitter and Facebook to get out news. Several Web sites have sprung up to monitor election irregularities, and the challengers are counting on voters to use their cellphones to announce results as they come in and to report abuses.

Dressed in a bright red suit at the final challenger rally, Nelson Chamisa, a candidate for Parliament, exhorted tens of thousands of party supporters to use their phones as a weapon for democracy. “If you have a cellphone, I want to see it,” Mr. Chamisa shouted.

Almost every hand went up, and a chant of “Show your phone!” washed over the crowd.

Mr. Chamisa later said that the well-attended rally was a sign that voters were fed up with Mr. Mugabe and not afraid to defy him. “This is the final nail in the coffin of dictatorship,” Mr. Chamisa said. “We are going to lower the coffin and bury it on Wednesday.”

The absence of violence in the days leading up to the election has emboldened many to openly support the main presidential challenger, Morgan Tsvangirai. Divisions within Mr. Mugabe’s party have also weakened his hand, with a succession struggle pitting the vice president against the defense minister in the race to succeed him.

“Mugabe’s own house is not in order,” said Pedzisai Ruhanya, a researcher at the Zimbabwe Democracy Institute, an advocacy group. “People feel they can come out and have their say in this election like never before.”