Turnout was reported to be high nationwide after queues of Zimbabweans lined up hours before polling opened on Wednesday to cast their ballots in a vote that could see Mugabe extend his 33-year rule.
The 89-year-old, Africa's oldest leader, is seeking a seventh term in office but his longtime rival Morgan Tsvangirai hopes the election will usher in a new era for the troubled southern African nation.
The African Union, which has been accused of whitewashing problems in the run up to the presidential and parliamentary vote, said initial reports indicated it was "peaceful, orderly, free and fair".
In Washington, US State Department spokesperson Marie Harf also said early signs indicated a "peaceful environment" – but that it was too soon to say if the election had been fair.
"We've made clear to the government of Zimbabwe and the region that further reductions in our sanctions will only occur if these next elections are credible, transparent and reflect the will of the Zimbabwean people," she told reporters.
It was the first election since the violent polls of 2008 that led to an uneasy power-sharing government between Mugabe and Tsvangirai as his prime minister.
There were no reports of widespread violence this time round, despite the fierce rhetoric of the campaign, although police said officers in Harare fired warning shots in the air and arrested 10 soldiers who tried to jump the queue to vote.
'Free and fair'
Many of the 6.4-million eligible voters started queueing before sunrise in the winter cold, hours before polls opened.
The lines continued well into the evening, with many marking their ballots by candle light.
Final results are expected within five days of election day and police warned on Wednesday that anyone trying to release unofficial figures ahead of time risked being arrested.
"I am sure people will vote freely and fairly, there is no pressure being exerted on anyone," Mugabe had said after casting his ballot.
The firebrand came to prominence as a hero of Africa's liberation movement, guiding Zimbabwe to independence in 1980 from Britain and white minority rule.
But his military-backed rule has been marked by a series of violent crackdowns, economic crises and suspect elections that have brought international sanctions and made him a pariah in the West.
On Tuesday Mugabe vowed to step down if Tsvangirai was the victor.
"If you lose you must surrender," he said.
Loyalists
As the economy recovers from a crisis that saw mass unemployment and galloping inflation, Mugabe loyalists insist their hero is "tried and tested" and dismiss concerns about his age.
Mugabe had focused his campaign on attacking homosexuals and on promises to widen the redistribution of wealth to poor black Zimbabweans.
Tsvangirai himself predicted that his Movement for Democratic Change (MDC) would win "quite resoundingly".
"This is a very historic moment for all of us," he said.
It is the time to "complete the change".
Tsvangirai won the first round of voting in 2008, but was forced out of the race after 200 of his supporters were killed and thousands more injured in suspected state-backed attacks.
Rigged?
​But the 61-year-old former union boss has repeatedly voiced concerns that the election was being rigged and the MDC on Wednesday listed a battery of alleged irregularities.
Finance Minister Tendai Biti, a senior MDC member, said the names of thousands of voters were missing from the electoral roll.
Speaking after a meeting with the electoral commission, Biti said: "They are admitting that there's still two-million people who are dead on the voters' roll, but they said 'because they're dead, they can't vote'."
The MDC handed its evidence to observers from the Southern African Development Community (SADC) and since no Western groups were allowed to monitor the polls, its account will be closely watched.
Tsvangirai hopes his plans to lure back foreign investors, create a-million jobs in five years and improve public services will deliver a long-awaited victory.
Turnout appeared to be particularly brisk in urban areas where Tsvangirai has enjoyed his strongest support.
But some analysts cautioned against interpreting the high urban turnout as a sign Tsvangirai would sweep the election – in which the victor needs more than 50% to avoid a second round.
"This election is going to be decided in the rural areas," where two thirds of Zimbabweans live and where Mugabe enjoys strong support, said Michael Bratton, founder of polling organisation Afrobarometer. – AFP