Harare: President Robert Mugabe faced the biggest challenge of his 28-year-rule as Zimbabweans began voting for a new president on Saturday.

Mugabe's biggest rivals in the race are veteran opposition leader Morgan Tsvangirai and ruling party defector Simba Makoni.

Tsvangirai has alleged that surplus ballot papers will be used to rig the vote.

The state-owned Herald newspaper reported on Friday that an opinion poll showed Mugabe would win up to 57 per cent of the vote.

If no candidate wins more than 51 per cent of the vote on Saturday, the election will go into a second round, when the two opposition parties would likely unite.