The runoff followed a disputed March 29 election that had opposition leader Morgan Tsvangirai ahead of Mugabe by 48 percent to 43 percent, according to the New York Times. But because neither candidate received 50 percent of the vote, a runoff was necessary.
Tsvangirai pulled out of the run-off on Sunday, citing widespread violence against his followers. Dozens of opposition supporters have been killed and thousands injured.
Several world leaders condemned the vote. U.S. Secretary of State Condoleezza Rice called it a "sham." EU spokeswoman Krisztina Nagy said the election result would be "hollow and meaningless," quoted the Associated Press.
Voting began Friday shortly after 5 a.m. GMT, and turnout was reportedly low compared to the March vote when people lined up at polling sites.
Paramilitary police in riot gear patrolled the city of Harare. Militant Mugabe supporters roamed the streets, sang revolutionary songs and asked people why they weren't voting, reported the AP.
People who voted had a finger marked with red ink.
"I've got no option but to go and vote so that I can be safe," said one woman selling tomatoes.
Tsvangirai told a news conference, "What is happening today is not an election. It is an exercise in mass intimidation with people all over the country being forced to vote."
He has taken refuge in the Dutch Embassy for several days.
A Mugabe supporter, Richard Munsaka, said the question of a free and fair election "depends on the eyes of the beholder," according to the BBC.
"I'm not saying there is no violence in the east of the country ... but not on a massive scale and that in itself cannot stop the whole country from going to vote just because a few individuals are kicking themselves," he said.
Mugabe, who has been president since independence in 1980, is believed to want a high turnout to claim an overwhelming victory over Tsvangirai, whose name still appeared on the ballot because he did not withdraw in time to remove it, election officials said.Mugabe, 84, voted in Harare. When a reporter asked how he felt, he said, "very fit, very optimistic, upbeat and hungry," according to the AP.