Morgan Tsvangirai claims he is heading for a “resounding” victory against president Robert Mugabe in Zimbabwe’s presidential election.
Casting his vote in the capital, Harare, Mr Tsvangirai said he believed he had the support to oust his old rival.
“It’s not if but when,” he said.
Zimbabweans began queuing outside polling stations before dawn with African observers reporting no major problems in the process, despite allegations of vote rigging and fraud.
Mr Mugabe has repeatedly pledged that the election will be free and fair.
“There is no pressure being exerted on anyone,” he told reporters as he voted.
The 89-year-old is seeking to extend his 33-year grip on power in the country, but has promised to “surrender” if the result goes against him.
But many have dismissed the president’s words and Mr Tsvangirai himself has said he takes the promise “with a pinch of salt”.
Suspicions of vote rigging were sparked by a Research and Advocacy Unit study, which said the electoral roll includes one million dead voters or people who have emigrated.
Voters appear keen to have their say though, with some queuing up wrapped in blankets fours hours before polling stations opened.
Several hundred people waited to vote in green tents in Mbare, the oldest township in Harare.
“I am happy to have cast my vote,” said Ellen Zhakata, 66.
“I just want an end to the problems in our country.
“All my children are outside the country because of the economic troubles here. I am so lonely. How I wish they could be working here.”
Zimbabwe’s economic problems have seen millions leave the country to find work in recent years.
Hyperinflation reached 231,000,000% five years ago but a scarcity of US dollars – now the country’s main currency – means the economy has stabilised and inflation is back in single digits.
Violence marred the last election in 2008, with Mr Tsvangirai pulling out of a second round run-off against Mr Mugabe after 200 of his supporters were killed.
The African Union and the the Southern African Development Community have both sent observers to monitor this year’s elections.
However, there are still worries that violence and intimidation could be used.
“We are very worried that if the result does not go Robert Mugabe’s way we could see a return to the violence that marred the 2008 election,” said Tiseke Kasambala, Southern Africa Director for Human Rights Watch.
She told Sky News: “The heads of his security forces have made clear that they are not going to accept a victory by the opposition or by Morgan Tsvangirai.”
The opposition leader has based his latest campaign on a plan to lure back foreign investors, create one million jobs in five years and improve public services.
Mr Mugabe has focused his promises on expanding the redistribution of wealth to poor black Zimbabweans, as well as making threats of violence against homosexuals.
Final results are expected in about five days.