With 36.6 per cent of ballots counted, Rowhani had 50 per cent of the vote, more than three times as many as his nearest rival, conservative Tehran mayor Mohammad Baqer Qalibaf on 15 per cent, said the interior ministry.
Under Iranian law, any candidate must clear a threshold of 50 per cent plus one vote to avoid having to face the second-placed candidate in a run-off, which would be held on June 21.
And voters reportedly turned out in massive numbers for yesterday's election, with Rowhani benefitting from the withdrawal this week of the only moderate candidate Mohammad Reza Aref.
Aref, a former first vice president, pulled out of the race on Tuesday at the urging of former reformist president Mohammad Khatami, who then threw his weight behind the 64-year-old moderate cleric.
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Rowhani, a former top nuclear negotiator who has vowed to mend Iran's ties with the international community, garnered nearly 6,049,655 votes from 12,091,699 ballots counted by 12:00 pm (1300 IST), the interior ministry said today.
During his tenure, in 2003, the Islamic republic agreed to suspend uranium enrichment. It was restarted after Ahmadinejad first became president in 2005.
Iran has been at loggerheads with world powers over its nuclear ambitions, which the West suspects is aimed at developing atomic weapons. The stand-off has resulted in the imposition of harsh economic sanctions and Tehran's international isolation.
On the votes counted so far, Rowhani is followed by Qalibaf with 15 per cent; ex-commander of the Revolutionary Guards Mohsen Rezai with 12.5 per cent; top nuclear negotiator Saeed Jalili with 11.5 per cent. All three hail from conservative camps.