With long lines at the polls Friday, voting hours were extended by five hours in parts of Tehran and four hours in the rest of the country. Turnout reached 75 percent, by official count, as disaffected members of the Green Movement, which was crushed in the uprising that followed the disputed 2009 presidential election, dropped a threatened boycott and appeared to coalesce behind a cleric, Hassan Rowhani, and the mayor of Tehran, Mohammad Baqer Ghalibaf.

Iran's interior minister, Mostafa Mohammad Najjar, said Saturday morning on state television that preliminary results showed Mr. Rowhani with a strong lead, followed by Mr. Ghalibaf. Mr. Najjar did not say when the final result would be available. Iran has more than 50 million eligible voters and as of early Saturday morning nearly five million votes had been counted.

The early results seemed to be a repudiation of the coalition of conservative clerics and Revolutionary Guard commanders, the so-called traditionalists, who consolidated power after the 2009 election, which the opposition said was rigged. The traditionalists' favored candidate, Saeed Jalili, Iran's chief nuclear negotiator and a protégé of the supreme leader, Ayatollah Ali Khamenei, did not seem to have gained much traction with the public, emphasizing vague concepts like "Islamic society" and standing up to Western pressure.

Early Saturday, officials at the Interior Ministry with access to the preliminary tallies said that Mr. Rowhani appeared to be the clear winner in some cities but that nothing had been confirmed. The ministry's early figures showed Mr. Rowhani with just more than 50 percent of the votes counted, news agencies reported. Early Saturday, Abbas Ali Kadkhodaei, the spokesman for the Guardian Council, warned against publishing any rumors and urged all to wait for the official results.

Nonetheless many veteran Iran political watchers, who had expected a conservative winner in what had been a carefully vetted and controlled campaign, expressed surprise.

"If the reports are true, it tells me that there was a hidden but huge reservoir of reformist energy in Iran that broke loose in a true political wave," said Cliff Kupchan, an Iran analyst for the Eurasia Group, a political risk consulting firm in Washington. "It was unpredictable — not even tip of the iceberg visible two days or three days ago — but it seems to have happened."

Farideh Farhi, an Iranian scholar at the University of Hawaii, while careful not to draw conclusions until the official result was known, said it was clear that reformists and other disaffected voters in Iran had summoned energy to mobilize for a heavy turnout despite their own doubts about the system.

"Everyone's assumption was they would not be able to create a wave of voters in the society," Ms. Farhi said. "This outcome was not something planned by Ayatollah Khamenei."

In surveys and interviews throughout the campaign, Iranians have consistently listed as their top priorities the economy, individual rights and the normalization of relations with the rest of the world. They also said they saw the vote as a way to send a message about their displeasure with the direction of the country, which has been hobbled by economic mismanagement and tough Western sanctions, stemming from the government's refusal to stop enriching uranium.

Mr. Rowhani, a former nuclear negotiator, had been criticized by Mr. Jalili for being too willing to bargain away Iran's nuclear program, which the West says is a cover for developing nuclear weapons but Tehran says is for peaceful purposes.

Rick Gladstone and Robert Mackey contributed reporting from New York.