Iranians vote in what could be the most crucial presidential election since the 1979 revolution.
Voting has begun in Iran's presidential election. It's expected to be a tight race between incumbent President Hassan Rouhani and former prosecutor, Ebrahim Raisi.
If no-one wins more than 50 percent of the vote, the top two candidates will compete in a run-off, scheduled to take place next Friday. More than 54 million people are eligible to vote.
Some 350,000 security forces were deployed around the country for protection during the election, state television reported.
Polls close at 6:00pm (1330 GMT), although authorities often extend voting into the evening. Ballot counting will start at midnight and final results are expected within 24 hours of polls closing, the semi-official Fars news agency said. The elections are also for city and village councils.
The vote is a choice between further support for the policies of the incumbent president or a protest that his policies have not delivered on promises made during his first term.
President Rouhani has worked throughout his four years in office to lift economic sanctions that are part of a deal with major world powers in return for curbing Iran's nuclear ambitions.
Raisi, however, has criticised the deal, accusing Rouhani of being "weak" in negotiations.
Ultimately, it is the country's Supreme Leader Ayatollah Ali Khamenei who holds sway over Iran's foreign policy.
While Rouhani appears to be the more popular candidate among the public, Raisi enjoys close ties with the Supreme Leader and backing from the country's powerful elite Revolutionary Guards.
"Everyone should vote in this important election ... The country's fate is determined by the people," Supreme Leader Ayatollah Ali Khamenei said, casting his ballot in Tehran as lengthy queues formed across the country of 80 million.
Rouhani has urged the Revolutionary Guards not to meddle in the vote in an effort to avoid a repeat of 2009's months of nationwide protests, which erupted amid suspicions that the Guards had falsified voting results in favour of former president Mahmoud Ahmadinejad.
But in an apparent reference to the 2009 disturbances, Khamenei has previously warned he would confront anyone trying to interfere in the election.
Economic problems, high unemployment and frustration over the slowness of reforms have plagued Rouhani's presidency, despite the gradual lifting of sanctions.
As part of his campaign, Raisi has been visiting rural areas and villages, promising housing, jobs and more welfare benefits for the poor.
Analysts have rejected Raisi's promises of jobs and cash handouts as unrealistic but admit these could win traction with voters who have felt few benefits so far from the nuclear deal.
In a televised debate, Raisi accused Rouhani of "economic elitism, mismanagement, yielding to Western pressure, and corruption."
Rouhani hit back in a sharper campaign strategy to mobilise Iranian women and young people who became jaded about the vote after losing hope in his ability to ease religious repression in society as promised in 2013, when he won by a landslide.