"The enthusiastic participation of Iranians in the election reinforces our national power and security," he said, as polling stations reported queues were far bigger than in last year's parliamentary election.
Rouhani has sought to frame the vote as a choice between greater civil liberties and "extremism".
But he faces stiff competition from hardline cleric Ebrahim Raisi, 56, who has positioned himself as a defender of the poor and called for a much tougher line with the West.
"We must all respect the vote of the people," Raisi said as he cast his ballot in southern Tehran.
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