The opposition Movement for Democratic Change (MDC) accused the ruling Zanu-PF of implementing a multi-pronged strategy to ensure that President Robert Mugabe emerged the winner of the 27 June run-off.
In the first round in March, which was relatively peaceful, with the MDC free to campaign around the country, the MDC's Morgan Tsvangirai gained about 120,000 more votes than Mr Mugabe, according to official results.
Since then, the MDC says a campaign of violence has been unleashed against its supporters to try to overturn that deficit in the second round.
Zanu-PF denies this, blaming the violence on the MDC in turn.
Faced with ongoing threats of the use of violence, the MDC decided on 22 June - just five days before the run-off - that its leader, Morgan Tsvangirai, was pulling out of the contest.
Here are the main complaints of the two parties:
The MDC, backed up by human rights groups, say their supporters have been attacked around the country, especially in rural areas which the opposition won in the first round of elections in March.
At least 70 have been killed and 25,000 forced to flee their homes, the MDC says.
They say Zanu-PF militias have systematically targeted key opposition activists to be beaten, abducted and have their homes set on fire.
Several reports, including documents obtained by the BBC, suggest that the military is behind the campaign.
The strategy would be to force MDC supporters out of their homes so they cannot vote and to scare potential opposition voters into backing Zanu-PF instead.
The army has denied these charges and Zanu-PF officials have accused the MDC of being behind the violence, while saying the scale has been exaggerated.
Some human rights groups have reported that MDC supporters have fought back when attacked by Zanu-PF militias.
MDC rallies have been banned, with the police citing security concerns.
Mr Tsvangirai has been arrested on several occasions when he tried to go on "meet-the-people" tours.
Morgan Tsvangirai campaigns by bus, as his rallies have been banned
Instead of holding large public rallies, he has been touring the country in a bus, getting out in villages and shopping centres.
President Robert Mugabe has meanwhile been campaigning around the country.
MDC Secretary General Tendai Biti has also been arrested and charged with treason.
The opposition said this was a deliberate attempt to block its campaign.
The state-controlled broadcaster ZBC refused to take campaign adverts from the MDC.
Justice Minister Patrick Chinamasa defended this by saying that the international media was biased in favour of the MDC and did not portray Zanu-PF's point of view.
Obviously for the election campaign, it is local media which counts.
Western observers were banned, as President Mugabe accused them of bias in favour of the MDC, which he says was set up by the former colonial power, the UK, to remove him from power.
The government also greatly reduced the number of local election observers it is accrediting, again accusing them of bias.
The main independent monitoring group, ZESN, had hoped to deploy 12,000 observers but was only allowed to send 500, to keep track of the 9,000 polling stations.
Its observers have been attacked and some killed.
ZESN released its own results from the first round and was accused of working with the MDC.
Some 500 observers from African organisations were on hand to monitor the poll.
The MDC also accused the government of using food aid as a political weapon - threatening to withhold food from areas that vote for the opposition.
Almost half the population may need food aid next year
The UN predicts that some five million people - almost half of the population - will need food aid by early next year, so this is a huge threat.
The government also temporarily banned aid workers from distributing food aid.
This was seen as a ploy to tighten its control of food aid, while removing any potential witnesses to the violence.
Zanu-PF accused the aid agencies of backing the MDC and distributing food aid as part of its campaign.
This was strongly denied by the aid agencies.
The MDC was also fearful of rigging and ballot box-stuffing on election day.
In the first round, it believes that it was cheated of outright victory by a mysterious block of 120,000 votes for President Mugabe.
The opposition says thousands of ghost voters remain on the voters' roll, who might nevertheless cast their ballots for Zanu-PF.
And there have been reports that Zanu-PF militants were being recruited as polling agents, to replace the teachers and civil servants who usually conduct the count.
Many teachers in rural areas were attacked and accused of using their positions to help the MDC in the first round.
Some were arrested and accused of electoral fraud.