Iranians are voting in the most keenly contested presidential election in years.
Polling stations across the Islamic Republic opened at 8am (03:30 GMT) on Friday and are due to close 10 hours later, the country's interior ministry said.
"We are expecting our dear citizens to come forward and vote in the early hours," the ministry said.
Opinion polls suggest that Mir Hossein Mousavi, a reformist candidate, is the closest challenger to Mahmoud Ahmadinejad, the incumbent in a four-person race.
But hours after voting got under way, Mousavi claimed that some of his representatives had been blocked from entering polling stations to monitor the vote.
He also accused the state-owned telecommunications provider of shutting down its system.
"Unfortunately, some of my representatives were blocked from entering polling stations and SMS [text messaging] is also down, which is against the law," Mousavi said after voting, according to his campaign website.
"We should not be fearful about the free flow of information, and I urge officials to observe the law."
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