Iran is holding its first elections since 2009. even as the leaders of its main opposition group remain under house arrest.
The poll to select 290 members of the parliament, the Majlis, is widely seen as a contest between President Mahmoud Ahmadinejad and the supreme leader Ayatollah Ali Khamenei.
The president and the ayatollah are both conservatives and profess roughly the same ideology. But analysts say their agreements mask deep antipathy, which developed after Khamenei supported Ahmadinejad in the 2009 poll.
Some have even compared the elections to the American Republican primaries, such is the fierce competition over relatively minor political differences.
Ahmadinejad's faction is thought to be more nationalist, and aimed at giving more power to the security forces and the government over the clergy.
The more theocratic Islamists led by the Ayatollah approve all parliamentary candidates through the Guardian Council.
The dispute between them was exposed last year when Ahmadinejad opposed Khamenei's choice of the head of the intelligence services.
Iran's parliament has powers ranging from setting the budget to advising on foreign affairs.
As he cast his vote in Tehran, Khamenei said that it was a "duty" for all Iranians to participate in the process.
"Because of the controversies over Iran and increased verbal threats," he said. "The more people come to the polling stations, the better for the country."
"The higher the turnout, the better for the future, prestige and security of our country … The vote always carries a message for our friends and our enemies."
Around 48m people are eligible to vote at the 47,000 polling stations around the country.
However many Iranians are expected to stay away from the polls, including some supporters of the president who have complained their candidates have not been allowed to stand by Khamenei's supporters.
Leaflets emerged online encouraging people not to vote, which were apparently distributed in the country.
The BBC said that the result of the election will have little impact on the country's nuclear ambitions, but could alter the balance of power ahead of the next presidential election in 2013.
If Ahmadinejad is seen to have lost it will cast severe doubt on his ability to survive the next poll as president.
The previous presidential poll in 2009 was hit by widespread accusations of voter fraud.
The leaders of the opposition Green Movement, Mir Hossein Mousavi and Mehdi Karoubi, have been under house arrest for more than a year, after they backed a spate of widespread protests.