HARARE, March 29 (Reuters) - President Robert Mugabe denied planning to rig Zimbabwe's elections on Saturday and said that although he was sure of winning he would be ready to accept defeat.

"We do not rig elections. We have that sense of honesty. I cannot sleep with my conscience if I have cheated in elections," he told reporters as he voted as a primary school in Harare.

"Why should I cheat? The people are there supporting us. The moment the people stop supporting you, then that's the moment you should quit politics."

Zimbabweans are voting in the most crucial election since independence from Britain in 1980, with Mugabe facing the biggest challenge of his 28-year-rule.

He faces a formidable two-pronged attack from veteran opposition leader Morgan Tsvangirai and ruling party defector Simba Makoni. Both accuse him of plotting to rig the poll.

If no candidate wins more than 51 percent of the vote on Saturday, the election will go into a second round, when the two opposition parties would likely unite. Critics say Mugabe will do his utmost, including rigging, to avoid this happening.

He said on Saturday that a second round was unlikely.

"We are not used to boxing matches where we go from round one to round two. We just knock each other out," said Mugabe.

"That's how we have done it in the past. That's how we will do it this time.

Asked how he rated his chances of winning, Mugabe said:

"Very good. I rate them the same way as in the past. We will succeed. We will conquer." (Reporting by Stella Mapenzauswa; editing by Michael Georgy and Barry Moody)

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