Iran's supreme leader Ayatollah Ali Khamenei has urged voters to go to the polls, accusing the country's "enemies" of encouraging a boycott.
Khamenei said Iranians must fulfil their duties
He was speaking as he voted in Iran's general elections, in which hardliners are widely expected to retake control of the 290-seat parliament.
Polling stations are to stay open for an extra hour, until 1900 (1530 GMT).
Reformists say a conservative comeback is inevitable after some 2,500 pro-reform candidates were disqualified.
The ban was decreed by the Guardians Council - the hardline vetting body.
The question dominating the election is how many of Iran's 46 million eligible voters will cast their ballots, the BBC's Stuart Hughes reports from Tehran.
There is widespread disillusionment with the reformists over their failure to liberalise the Islamic state, he says.
Iranian state radio said the extra hour of voting had been ordered because of a high turnout.
The Guardians Council deemed many candidates ineligible because of their alleged indifference to Islam and to the constitution, or accused them of questioning the supreme leader's powers.
But critics say the process has become a means by which the Council eliminates rivals.
The tens of thousands of venues for voting included mosques, desert outposts for nomads and cemeteries for those making the traditional weekly visit to graves.
But the biggest of the reform parties, the Participation Front, is not contesting the poll after many of its top members were disqualified.
Other reform factions closer to the centre of the political spectrum are taking part, however.
A recent government survey predicted a turnout of about 30%.
An exiled Iranian opposition group says reports from hundreds of polling venues on Friday suggest low turnouts across the country.
The National Council of Resistance of Iran told BBC News Online that "at some sensitive voting centres there are dozens of security agents".
The test for the reformers led by President Mohammad Khatami is whether a disillusioned electorate will support the few factions who are fielding candidates, or turn away from them as traitors, the BBC's Jim Muir in Tehran says.
The general expectation is that the conservatives will win a majority but still leave moderate reformists with a voice, he adds.
At one polling station in Tehran, a group of about 20 girls, all dressed in black chadors, waited to
"I'm voting for the first time," said 16-year-old Sara Nazari. "This is a very important moment for the
Mr Khatami, described as looking gloomy, voted at the Interior Ministry and told reporters: "This nation has been defeated many times but continued its path and created surprises."
'Traitors to Islam'
On the eve of the election, some of the country's best-known intellectuals and journalists called for a boycott, accusing the hardliners of widespread clampdown.
But Ayatollah Khamenei told state television in Tehran such people were "against the Iranian nation and the revolution".
Ahead of the poll there was little sign of public interest
Ayatollah Ahmad Jannati, head of the Guardians Council, said those calling for a boycott were "traitors to Islam and their country".
He told worshippers in a Friday prayer sermon, that each ballot cast would be akin to "firing a bullet into the heart of [George] Bush".
Conservatives say the incumbent administration has only itself to blame for voter apathy.
The government led by President Khatami had squandered its term "creating commotion, tension and despair", the hardline daily Jomhuri-ye Eslami wrote earlier this week.