"I can't vote for the people who have brought me into this state", said the 94 year old who ruled Zimbabwe for nearly four decades before resigning in November amid the threat of a military takeover and impeachment.
Nicknamed "the Crocodile", an animal famed in Zimbabwean lore for stealth and ruthlessness, Mr Mnangagwa, of the Zimbabwe African National Union-Patriotic Front (ZANU-PF), has been president since the military ousted Mr Mugabe in a bloodless coup.
I think it is just Chamisa,"he said". He acknowledged that there was a "good turnout".
"Things have been tough", Tawanda Petru, 28, an unemployed man voting in Mbare, a low-income district of the capital Harare, told AFP as polling stations opened across the country.
Mnangagwa voted with his family in his hometown of Kwekwe in the Midlands province.
Global observers from the European Union, the United States and the Commonwealth are watching polling stations across the country.
Elections during Mugabe's authoritarian rule were often marred by fraud and violence, and this year's campaign has also been dominated by accusations that the vote would be rigged.
In order to understand the significance of this vote, we need to look back at the recent history of the country.
The country is expecting a high turnout of first-time voters. More than 5.6 million people registered to vote for the president, 350 lawmakers and local government representatives.
Polls close in historic Zimbabwe election
A record of more than 20 presidential candidates and almost 130 political parties were participating. If no presidential candidate wins 50 per cent of the vote, a runoff will be held September 8.
"People finally got to say who they want, what they want", said Elton Ashely, 23, a student who served as a polling agent for the MDC in Mbare. I am not leaving anything to chance. First violation will result in booking of case and attract Rs 25,000 penalty, whereas the second time Rs 50,000 fine would be slapped.
Chamisa, a 40-year-old lawyer and preacher, is facing off against 75-year-old President Emmerson Mnangagwa, a former intelligence chief and one-time ally of Mugabe.
In Zimbabwe's second largest city of Bulawayo, another presidential candidate Thokozane Khupe, a leader of a break-away MDC faction said she was confident of victory but expressed concern that voting queues were moving at a snail's pace.
Chamisa voted in Kuwadzana, a poor suburb in Harare where he has been an MP for the past 18 years.
The MDC was able to access rural districts that were once no-go areas, but it's still skeptical the election will be fair.
Speaking to journalists he said Mugabe who dumped his party live on camera, is free to engage with him any time.
"Zimbabwe's economy requires commercial bridge loans, donor support, relief on its huge dollar-denominated debt and, ultimately, an International Monetary Fund program", analysts at Exotix said in a research note on Monday. "They are doing this bidding on behalf of Mnangagwa who is panicking", he said.
"After years of stasis the events of November 2017 gave Zimbabwe the chance to dream again", Mnangagwa said on Sunday in an address on state radio.
"However the public's faith in the secrecy of the ballot is essential for the credibility of the process". He walked unaided into the polling station at a primary school in the Highfield district of Harare, accompanied by his wife Grace.
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