"I can not vote for those who have tormented me", Mugabe said in a reference to Mnangagwa, who took office with the military's support. "He is young and can understand our plight as youth", said Ndumiso Nyoni, 20, a worker at a lodge in Lupane, southern Zimbabwe.
They are running for the presidency for the first time, and need more than 50% of the vote to win outright.
Estimated turnout was around 75 percent before polls closed on Monday evening after a peaceful day of voting.
The Zimbabwe Electoral Commission (ZEC) says vote counting and verification is at various stages and people should expect the announcement of results from around 3.pm.
"There are shortcomings that we have to check".
Residents of Mbare township examine uncollated results posted on a polling station following general elections in Harare, Zimbabwe, July 31, 2018. "Because people are not yet settled, they're thinking of too many things".
Zimbabweans are also electing 210 members of parliament and more than 9,000 councillors.
The victor faces the task of putting Zimbabwe back on track after 37 years under Mugabe, tainted by corruption, mismanagement and diplomatic isolation that caused a crisis in a country that once had one of Africa's most promising economies.
Ex-prime minister of Ethiopia, Hailemariam Desalegn, who is the head of the African Union Election Observer Mission in Zimbabwe, told reporters that there has been a high voter turnout.
Verryn said the historic day had been remarkable.
"I am scared - is there going to be unrest?" "It was extremely remarkable, dignified and well-organised", he said.
How were candidates received at polling stations?
Mugabe emerged on the eve of the election to announce he would vote for the opposition, surprising Mnangagwa who accused him of striking a deal with Chamisa.
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"There seems to be a deliberate attempt to suppress and frustrate" urban voters through "unnecessary delays", Chamisa said on Twitter.
The EU had not yet concluded how to judge the vote, he said.
The victor of this landmark election will be tasked with putting Zimbabwe back on track after 37 years of Mr Mugabe's rule.
Mnangagwa, leader of the ruling ZANU-PF, cast his vote far outside of Harare, and appeared relaxed and confident.
Justice Chigumba said voter turnout at polling stations countrywide ranged between 60 and 78%.
"I am very happy that the process for campaigning was peaceful [and] voting today is peaceful", he said. As voters race to cast their ballots, some prophets are predicting the outcome of the first election without Mugabe.
Opinion polls give the 75-year-old former intelligence chief only a slim lead over Chamisa, 40. "President! The president is here!"
Support for Zanu-PF historically has been deepest in rural areas, particularly the party's Mashonaland heartland, where more than two-thirds of Zimbabwe's 17 million people live.
Mugabe did not answer, when asked by VOA, for whom he voted.
A woman talks on the phone after casting her vote at a polling station.
"We may have made some mistakes but I think President Mnangagwa is the only person who has the interests of Zimbabwe at heart".
Zimbabwe's much-criticised election authority declared Tuesday that the vote had been free of rigging - even though the count was not yet completed.
Wearing a red tie, red pocket handkerchief and dark suit, Mugabe had his finger marked with a pen to show he had voted and then spend several minutes in the cardboard polling booth selecting his choices on the three ballot papers.