Counting has begun in Zimbabwe in the first election since the removal of former president Robert Mugabe, a watershed vote that could welcome a pariah state back into the international fold and spark an economic recovery.

However the opposition leader has alleged voter suppression, raising fears of a disputed result.

Nelson Chamisa is the main challenger to President Emmerson Mnangagwa, who served for decades under Mr Mugabe.

Mr Mnangagwa is viewed as the frontrunner, though the latest poll shows a tight race.

The election winner faces the task of putting Zimbabwe back on track after 37 years of Mugabe rule tainted by corruption, mismanagement and diplomatic isolation that caused a crisis in a country that once had one of Africa's most promising economies.

Mr Chamisa has said the Zimbabwe Electoral Commission (ZEC) was impeding voting in urban areas where he enjoys strong support but gave no evidence to back the claim.

"The people's will being negated and undetermined due to these deliberate and unnecessary delays," he said in a tweet.

Emmerson Mnangagwa cast his ballot at Sherwood Primary School in Kwekwe

The ZEC was not immediately available to comment. It has denied Mr Chamisa's previous allegations of bias.

Mr Chamisa said his Movement for Democratic Change (MDC) would win if there was no "ballot mischief", making it likely he will challenge the outcome if Mr Mnangagwa prevails.

A contested result is likely to lead to street protests and possible violence.

Dozens of people were killed ahead of a runoff in 2008 between Mr Mugabe and MDC-founder Morgan Tsvangirai, who died of cancer in February.

A credible vote is essential if Zimbabwe is to exit painful sanctions and secure the donor funding and investment needed to stem chronic cash shortages.

As a result, the views of election observers are seen as crucial.

Emmerson Mnangagwa (left) and Nelson Chamisa

EU chief observer Elmar Brok said many voters, particularly young women, left voting queues in frustration at long delays.

The EU has not yet concluded how to judge the vote, he said.

"In some cases it works very smoothly but in others we see that it is totally disorganised and that people become angry, people leave," Mr Brok told reporters in Harare, adding that it was unclear whether the problems stemmed from coincidence or bad organisation.

Mr Mugabe, 94, led the war for independence and since 1980 has dominated the country's politics.

But since he stepped down under military pressure in November he has lived in obscurity.

He emerged on the eve of the election to announce he would vote for the opposition, surprising Mr Mnangagwa who accused him of striking a deal with Mr Chamisa.

Robert Mugabe casts his vote

Mr Mugabe made no comment to reporters as he cast his ballot around lunchtime accompanied by his wife, Grace.

A huge crowd gathered outside, some cheering, many booing.

Mr Mnangagwa denied Mr Mugabe's claim that the vote would not 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," Mr Mnangagwa told public television after voting.

Opinion polls give the 75-year-old former intelligence chief only a slim lead over 40-year-old Mr Chamisa.

There will be a runoff on 8 September if no candidate wins more than half the votes.

Mr Mnangagwa, of the Zimbabwe African National Union-Patriotic Front (ZANU-PF), has been president since the military ousted Mr Mugabe.

Elections under Mr Mugabe were often marred by intimidation, rigging and violence but the consensus is the build-up to this vote has been better than before, although Mr Chamisa complained about a flawed voters' roll and opaque ballot paper printing.

Mr Chamisa, a lawyer and pastor, has attracted young and unemployed voters frustrated with nearly four decades of ZANU-PF rule.

"Victory is certain, the people have spoken," Mr Chamisa said after casting his ballot in Harare as a cheering crowd chanted: "President! President! The president is here!"

"I have no doubt that by end of day today we should be very clear as to an emphatic voice for change," Mr Chamisa said.

Polling stations opened at 7am local time, and were due to close at 7pm. Zimbabweans are also electing 210 members of parliament and more than 9,000 councillors.

The European Union, the United States and the Commonwealth have sent observers.

Mr Mnangagwa has made a big effort to win over foreign opinion; hosting Western ambassadors, courting investors and patching up relations with white commercial farmers who were violently evicted from their farms under Mr Mugabe.