Zimbabwe's President Robert Mugabe, Africa's oldest leader at 89, said on Wednesday that he will serve a full five-year term if re-elected into office.

Mugabe told reporters after casting his vote at a primary school in south Harare that he would be cheating his supporters if he retired before finishing the five-year term of presidency.

"Why do I offer myself as a candidate if it is to cheat the people by resigning soon after?" Mugabe said.

Mugabe, who has ruled Zimbabwe for 33 years, will turn 94 when he finishes his sixth presidential term by 2018, if he is re-elected.

Zimbabwe passed a new constitution this year, which for the first time stipulates that a person can only serve two five-year terms as the president. But it is not retroactive, meaning in theory that Mugabe can rule until 2023 if elected twice in a row.

For years, Mugabe has brushed aside speculations about his health, insisting that he is in shape to lead the country for another five years. His major challenger in polls is Prime Minister Morgan Tsvangirai, who tried but failed twice to unseat Mugabe over the past decade.

Tsvangirai had cast his vote earlier on Wednesday morning, calling it "historic" as he regards it a delayed presidential run-off from the disputed 2008 polls.

"Today is a very historic moment for us. It is an emotional moment, after all the conflict, suspicion and hostility, there is a sense of calmness," Tsvangirai told journalists just before he left the polling station.

Tsvangirai won the first round in the 2008 presidential race but pulled out from the run-off, citing widespread violence against his supporters.

He is admired by reformists in the country for his sheer courage and determination to usher in democratic change in the country.

Polls opened at 7am and will close at 7pm, but queuing at the polling stations began in the wee hours of the morning.

During the one-day vote, some 6.4-million voters will elect a president, more than 200 members of parliament and thousands of local councillors at 9 735 polling stations spread across the country.

According to the Zimbabwe Electoral Commission, a vote count will immediately commence after polls close. Results for the final presidential vote will be announced within five days. If no candidate wins 50% plus one vote, which is required for an outright win, a run-off will be held on 11 September.