Mugabe faces the biggest challenge to his 22-year rule
Reginald Matchaba-Hove, chairman of the Zimbabwe Election Support Network (ZESN) - a coalition of non-governmental organisations - said: "The election is total confusion and chaos... there is no way these elections can be described as substantially free and fair."
These give President Robert Mugabe roughly a 69,000-59,000 lead over his main challenger, Morgan Tsvangirai.
A leading MDC official, meanwhile, has been charged with treason in connection with an alleged plot to kill Zimbabwean President Robert Mugabe.
'Malicious propaganda'
Thousands of heavily-armed soldiers and police were deployed as counting got under way on Tuesday, following a third unscheduled day of voting.
Click here to tell us your experiencesBritain, which has been pushing for sanctions against Mr Mugabe's regime, said there was "pretty strong evidence" that President Mugabe had "stolen" the election, while Norway's observer mission said the elections failed to meet international standards.
But Zimbabwean Justice Minister Patrick Chinamasa said that anyone criticising the election was "spreading malicious propaganda".
Threat of violence
Tension has been high since the polls closed following a 24-hour extension won by the MDC, with observers and opponents of the government warning of violence.
Mr Matchaba-Hove said that "a flawed electoral process is a potential cause of conflict", and urged the public "to remain calm but firm, resolute even after the election results are released.
Anticipating an outbreak of trouble, the government positioned armed police and soldiers in cities, villages and at strategic sites in the Midlands province.
Residential suburbs have been sealed off and a curfew has been imposed from 1800 local time (1600 GMT) on Tuesday.
The first results were expected later on Tuesday, and the final outcome of the election will be known on Wednesday.
The MDC has warned of an "expression of anger" and said it will challenge the result in court if its candidate, Morgan Tsvangirai, is not declared the winner.
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Correspondents say that, as there were no exit polls, there is no indication whether Mr Mugabe has survived what is the stiffest challenge so far in his two decades as president.
State-run media has been suggesting Mr Mugabe is set to defeat Mr Tsvangirai and the three other candidates for president.
As counting began, a court in the capital, Harare, charged the Secretary General of the MDC, Welshman Ncube, with treason.
He had previously been arrested in February, along with Mr Tsvangirai and another party official, when police accused all three of treason.
Mr Ncube, who was released on bail, said his arrest was an act of political desperation.
"We remain firmly confident [of victory] otherwise they [the government] would not be in such a state of panic," he told the Associated Press news agency.
Voters 'turned away'
Preliminary figures suggest a turnout of nearly 70% in areas traditionally loyal to Mr Mugabe, and just below 50% in Harare, which the MDC considers its stronghold.
But election observers say many people did not have a chance to cast their votes before polls closed on Monday night.
The ZESN said monitors had confirmed problems at four polling stations when voting was stopped.
In the Harare suburb of Glen Norah, police wielding batons fired tear gas to disperse 600 people waiting to vote.
When ordered to go home, they began chanting "Change, change, we want to vote!"