Chamisa, 40, becomes the Movement for Democratic Change’s electoral champion after veteran leader Morgan Tsvangirai died of cancer on February 14 at the age of 65.
Tsvangirai embodied opposition to former president Mugabe whom the MDC accused of vote-rigging, voter intimidation and authoritarian behaviour.
But Tsvangirai’s final months were marked by increasingly public quarrelling between his deputies over who would succeed him as leader.
This year’s polls will be the first since independence in 1980 not to feature Mugabe.
Chamisa, who trained as a lawyer, rose through the MDC’s ranks as a leader of its youth wing. He served as a minister between 2009 and 2013 in a rocky government of national unity.
But he and the MDC face an uphill battle to defeat the ruling party’s candidate, President Emmerson Mnangagwa, Mugabe’s former deputy who succeeded him after his resignation following a brief military takeover in November.
The MDC — which has a history of splintering — appealed for unity as it announced Chamisa’s candidacy.
Party spokesman Luke Tamborinyoka called on “all leaders (to) stop communicating party positions, without permission… for purposes of message discipline and consistency”.
Fresh divisions, say analysts, could mean that Mnangagwa faces little opposition in the election at which, according to the country’s electoral commission, some five million people are registered to vote.
Fifty-four percent of those registered to cast a ballot are women, said Zimbabwe Electoral Commission (ZEC) chief Joyce Kazembe at a news conference in Harare Thursday, while 43.5 percent of eligible voters are aged 18-34. Zimbabwe has a population of around 16 million people.
“Voter registration is still on-going. Let us keep encouraging those who are yet to register to visit their nearest ZEC office,” said Kazembe, adding that the commission was “committed to credible, free and fair elections.”
Mugabe was regularly accused by the international community and local opposition parties of stealing elections by intimidating his opponents and suppressing voter turnout in areas hostile to his iron-fisted rule.
Mnangagwa, 75, has vowed to deliver “free, fair and credible” elections in Tsvangirai’s honour.