TEHRAN, Iran — Iranians packed polling stations from boutique-lined streets in north Tehran to conservative bastions in the countryside today with a choice that's left the nation divided and on edge: keeping hard-line President Mahmoud Ahmadinejad in power or electing a reformist who favors greater freedoms and improved ties with the United States.

Turnout was massive and could break records. Crowds formed quickly at many voting sites in areas considered both strongholds for Ahmadinejad and his main rival, reformist Mir Hossein Mousavi, who served as prime minister in the 1980s and has become the surprise hero of a powerful youth-driven movement. At several polling stations in Tehran, mothers held their young children in their arms as they waited in long lines.

Voting was extended by six hours to midnight.

The fiery, monthlong campaign unleashed passions and tensions. The mass rallies, polished campaign slogans, savvy Internet outreach and televised debates more closely resembled Western elections than the scripted campaigns in most other Middle Eastern countries.

President Barack Obama said Iran's "robust debate" leading up to elections shows change is possible there, and it could boost U.S. efforts to engage Tehran's leadership.