Harare - Zimbabwe's main opposition candidate, Nelson Chamisa, said there was an attempt to "suppress and frustrate" the vote at Monday's election in urban areas where his Movement for Democratic Change (MDC) party has strong support.
Chamisa also said the MDC would win the election unless there was rigging in rural regions, making it likely he will challenge the outcome if his rival, President Emmerson Mnangagwa, wins.
Zimbabweans voted on Monday in the first election since the removal of former president Robert Mugabe, a watershed moment they hope will rid the country of its pariah status and spark a recovery in its failed economy.
The election pits 75-year-old Mnangagwa, a long-time Mugabe ally, against 40-year-old Chamisa, a lawyer and pastor vying to become Zimbabwe's youngest head of state.
On the eve of the election, Mugabe emerged from eight months of obscurity since the military ousted him in a bloodless coup, to announce he would vote for the opposition, surprising former ally Mnangagwa who accused him of striking a deal with Chamisa.
After casting his ballot in the central city of Kwekwe, Mnangagwa was asked about Mugabe's claim that the vote wouldn't be free since it was being run by a "military government".
"I can assure you that this country is enjoying democratic space which has never been experienced before," Mnangagwa told public television outside the polling booth.
"In any democratic space and country, people have the freedom to express their views, negative or positive."
Opinion polls give the former intelligence chief, who took over as president after the army ousted Mugabe, only a slim lead over Chamisa. There will be a runoff on September 8 if no candidate wins more than half the votes.