The election was considered too close to predict. First results were expected early Saturday.
The first round winner, Ayatollah Hashemi Rafsanjani, had strong support from progressive and business groups. His surprise opponent, Tehran's hard-line mayor Mahoud Ahmadinejad, was backed by Iran's impoverished classes and powerful forces opposed to relaxing Iran's Islamic regime.
"This is the beginning of a new movement," Ahmadinejad declared after casting his ballot.
Early turnout appeared strong. About 63 percent of Iran's nearly 47 million voters cast ballots in the first round.
Election overseers warned the elite Revolutionary Guards and its vigilante wings, both key Ahmadinejad followers, to stay clear of polling sites following accusations of intimidation and other abuses in last Friday's balloting.
The two rivals represent a distinct vision and voice for a nation struggling to find its priorities. The race also exposed the estrangement between those who feel empowered and those who feel embittered by Western-friendly reforms that began in the late 1990s.