TEHRAN — Iranian voters turned out in huge numbers on Friday, a late surge of interest in the presidential vote that seemed to swing the tide in the favor of the most moderate candidate in the field. But it was uncertain whether any single contestant would exceed the 50 percent threshold needed to avoid a runoff next week.

With long lines at the polls, 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 three 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.