Zimbabwe is heading to presidential polls in a tense atmosphere with incumbent president, Robert Mugabe, confident of extending his 33-year rule.

Around 6.4 million Zimbabweans are eligible to cast their vote amid widespread expectations of a high turnout in the landlocked southern African country.

Election campaigning in the run-up to the election has been calm without any reports of violence.

"We have won already. It's a walk-over. This year nobody was beaten, or forced to attend any meeting, but the numbers of people at rallies say it all," a Mugabe supporter, Jestara Mziwanda, told AFP.

Voting began at 07:00 local time (05:00 GMT) and polling stations close at 19:00 local time. The results will be announced five days after polling. Scores of people have already queued up outside polling stations.

"I believe we ran a successful campaign. I believe the campaign also showed us that the people still support the principles of Zanu-PF [Mugabe's party] and believe in Zanu-PF. I want, therefore, as I thank them to wish all those who are candidates on our behalf that is standing on our behalf every success," 89-year-old Mugabe told reporters ahead of the polling.

The last presidential election which took place in 2008 was marred by violence. The political ramifications of a victory or defeat for Mugabe will be in the minds of Zimbabweans while casting their vote, according to analysts.

The election has been tainted by allegations of rigging by the ruling administration.

Mugabe is being challenged by three other contenders including sitting prime minister Morgan Tsvangirai.

Tsvangirai is facing Mugabe yet again in this election following the 2008 election, in which he won the first round.

Mugabe and Tsvangirai have been sharing the administration since 2009 following a deal brokered by the regional African bloc.

Tsvangirai has been using Mugabe's advancing age as a major campaign point.

"How can you let an old man push a plough when there are young people around? I want Mugabe to enjoy his retirement in peace and quiet," Tsvangirai said in his final rally in the capital Harare, which drew huge crowds.