Zimbabweans are suffering the world's highest inflation of more than 100,000 percent, chronic shortages of food and fuel, and an HIV/AIDS epidemic that has contributed to a steep decline in life expectancy.
And although the odds seemed stacked against Mugabe, in power since independence in 1980, analysts believe his iron grip on the country and solid backing from the armed forces could enable him to ignore the results and declare victory.
He rejects vote-rigging allegations.
The U.S. State Department called on Zimbabwe's electoral commission to put aside "partisan sympathies" and "follow the letter and spirit of the law".
Marwick Khumalo, head of an observer group from the Pan-African parliament, said the elections themselves were free, fair and credible overall.
But he added: "The mission is concerned that two days after the closure of the polls, the overall outcome of the elections remains unknown."
Official results so far showed ZANU-PF with 31 seats, MDC with 30 and a breakaway MDC faction with five.
The MDC said its tally showed it had won 96 parliamentary constituencies out of 128 counted. Makoni had 10 percent of the unofficial presidential vote count.
The MDC said unofficial tallies showed Tsvangirai had 60 percent of the presidential vote, twice the total for Mugabe, with more than half the results counted. Private polling organisations also put Tsvangirai well ahead.
"In our view, as we stated before, we cannot see the national trend changing. This means the people have spoken, they've spoken against the dictatorship," MDC Secretary-General Tendai Biti said.
In his first public comments since the vote, Makoni criticised the way results were being announced. "We are very worried by the manner in which things are unfolding," he said.
Tsvangirai and some international observers accused Mugabe of stealing the last presidential election in 2002.
Justice Minister Patrick Chinamasa and Public Affairs Minister Chen Chimutengwende both lost their seats.
The government has warned that any early victory claim would be regarded as an attempted coup.
