Iran Votes in Leadership Showdown
Iranians are voting in an unprecedented second round presidential run-off which offers the electorate two distinctly different visions for the future.
Contesting the election are former President Akbar Hashemi Rafsanjani and Tehran Mayor Mahmoud Ahmadinejad.
Mr Rafsanjani is a cautious reformer seeking a rapprochement with the West, Mr Ahmadinejad a hardline conservative.
The interior ministry says it has received reports of vote fraud from six polling stations in the capital.
It has also heard complaints from elsewhere in the country that unnecessary people are inside and outside polling stations.
The previous round was tarnished by allegations of fraud and intimidation, with three of the candidates alleging an orchestrated plot to influence the outcome in favour of the hardliners' candidate.
The ministry extended voting by one hour - to 1530 GMT - because a number of people were still waiting to cast their ballots after the official closing time.
An interior ministry spokesman said on Friday that he had received many reports of a systematic build-up of people inside and outside polling stations who had nothing to do with administering the process.
And one man, said to be an election observer from Mr Rafsanjani's camp, has been stabbed in a polling station in Tehran by unknown people.
The BBC's Frances Harrison in Tehran says the ministry has written to all of the country's governors, advising them to stop voting in places where violations are reported so they can investigate the allegations. READ MORE
She adds that there is no independent confirmation of the allegations of rigging, and government bodies in charge of the polls are themselves divided along political lines.
Ahead of Friday's poll, outgoing President Mohammad Khatami urged government departments to ensure the election was free and fair.
Mr Khatami told Iran's student-run news agency, Isna, that a "climate of terror" was being created around the vote.
Some 25 people have been arrested on suspicion of vote-rigging in the first round.
Many critics of the regime have already dismissed the polls as fundamentally flawed, given that all candidates have been pre-screened by the Guardians Council.
Mr Ahmadinejad's strong showing a week ago surprised many Iranians, when he beat five other candidates to go through to Iran's first second round run-off.
Supreme leader Ayatollah Ali Khamenei cast his ballot soon after polls opened at 0900 (0430 GMT).
"The more people who participate in the election, the better it will be for the next president and for protecting Iran, and achieving our goals," he said.
Opinion polls suggest the two candidates are neck and neck.
As Mr Rafsanjani voted, he told reporters: "It's a very close competition, but according to the information I have, I am ahead of the other candidate."
Mr Ahmadinejad said: "Today is the beginning of a new political era for the Iranian nation."
The differences between the two candidates have offered a stark choice to Iran's 47 million voters, half of whom are aged under 25.
Mr Rafsanjani is a former president who has campaigned for a new term in office promising social reforms and closer relations with the West.
Mr Ahmadinejad, by contrast, is a former military figure who has pledged to redistribute wealth and step up efforts to counter Western "decadence" within Iran's Islamic society.
Our correspondent says the choice between the two has divided the country from top to bottom along ideological and class lines.
While Mr Ahmadinejad's Islamic orthodoxy has concerned many, others have attacked Mr Rafsanjani's alleged wealth, branding him "a new Shah".
The bitterly-fought campaign has polarised Iranian society, exposing economic frustrations yet to be resolved more than 25 years after the Islamic revolution.
"I will vote for Ahmadinejad because he wants to cut off the hands of those who are stealing the country's national wealth. He wants to fight poverty, fraud and discrimination," said Rahmatollah Izadpanah, 41, queuing to vote in south Tehran.
But Reza Khatibi, 47, a book store owner, said: "Rafsanjani can manage the important issues of Iran... in a moderate way. If he's not elected, I will leave this country. It will be so dangerous."