The presidential race had been considered too close to call. Mr. Mnangagwa, 75, the incumbent who seized power from Mr. Mugabe and is the candidate of the ruling ZANU-PF party, is hoping that a win in a free and fair election would bring him personal legitimacy and economic assistance for Zimbabwe from the West.

For decades, Mr. Mnangagwa acted as Mr. Mugabe’s enforcer and played a leading role in rigging past elections. But as president, he has differentiated himself from his predecessor by reaching out to Western officials and investors, especially the British, as well as to white Zimbabweans.

The main opposition candidate, Mr. Chamisa, 40, outmaneuvered older and more experienced rivals to win the leadership of the Movement for Democratic Change Alliance after the death this year of its longtime leader, Morgan Tsvangirai, who ran twice against Mr. Mugabe. A skilled orator, Mr. Chamisa, who is also a pastor, energized supporters at scores of rallies across Zimbabwe in recent months.

Mr. Chamisa has made statements casting doubt on the integrity of the electoral process. A couple of days before the vote, he said that a victory by Mr. Mnangagwa would be “fiction” — despite the fact that a poll by Afrobarometer, a nonpartisan research organization, had shown him slightly behind Mr. Mnangagwa.

In a tweet on Monday, Mr. Chamisa accused election officials of causing delays to suppress the vote in the opposition’s urban strongholds. “The people’s will being negated and undermined due to these deliberate and unnecessary delays,” he wrote, though he offered no evidence.