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."
Mahdi Karroubi, a reformist and former parliamentary speaker, and Mohsen Rezai, a former commander of the Revolutionary Guard, Iran's elite military force, are also standing in the election.
Interest in the election among the 46.2-million electorate is so high that voting could be extended until midnight.
Al Jazeera's Alireza Ronaghi, reporting from the city of Isfahan, said that queues of people started building up at the gates of polling stations minutes before they were allowed in.
"By the look of things, there will be a very strong turnout [by the electorate], unlike last time," he said.
"It is pretty much neck-and-neck [between Ahmadinejad and Mousavi] in Tehran and other major cities.
"Even in Isfahan, where in the last election the whole city was behind Ahmadinejad, this time there seems to be a divide between the youth and the educated, who support Mousavi, and the less well-off and those living in the traditionally religious part of the city, who support Ahmadinejad."
Ayatollah Ali Khamenei, Iran's supreme leader, said that it was a "right and [a] duty of the people" to vote, as he cast his ballot.
"I urge people to arrive at the polling stations from early hours to vote and have their share in choosing the highest management of the country," he said.
Observers doubt that a change of president will bring major alterations to Iran's foreign policy or its alleged pursuit of a nuclear weapons capability as the policies are largely controlled by unelected religious leaders.
But in the final hours of campaigning, candidates sought to damage the image of their opponents.
|Mousavi has support among the younger members of the electorate [AFP]
Mousavi, a former prime minister, accused Ahmadinejad of isolating Iran with his vitriolic attacks on the US and said he lied about the country's economy.
But Ahmadinejad accused his rivals of using "Hitler-style" smear tactics and said they could face jail for insulting the president.
"Such insults and accusations against the government are a return to Hitler's methods, to repeat lies and accusations ... until everyone believes those lies," the semi-official Fars news agency quoted Ahmadinejad as saying.
Insulting senior officials is a crime in Iran punishable by a maximum of two years in prison.
Even after the official end of campaigning on Thursday, tens of thousands of Mousavi supporters remained in the streets, dancing on cars, waving green flags and handing out pro-Mousavi fliers.
The four candidates for the presidency were cleared by Iran’s Guardian Council, a panel of six senior clerics and six Islamic jurists
The council, which disqualified the rest of the 475 potential candidates who registered, bars women from standing
All Iranians aged over 18 can vote, which means 46 million of Iran's more than 70 million people are eligible
If no candidate wins at least 50 per cent plus one vote of all ballots cast, including blank ones, a run-off round between the two leading candidates will be held on the first Friday after the election result is declared
Trita Parsi, the president of the Iranian-American Council, told Al Jazeera that Ahmadinejad's attacks on Mousavi "seem to have backfired and may have motivated the youth to come out and vote, supporting Mousavi's platform of change".
Afshin Molavi of the New America Foundation told Al Jazeera that accusations against Ahmadinejad and the ruling elites of corruption and insider dealings "will continue to hang in the air long after these elections, and many Iranians know this about the ruling elites".
He said that while Iran has traditionally had high voter turnout, "when we're seeing so many voters than previous polls, it tends to reflect a switch to reformist candidates".
Iran's reformists are hoping that a high turnout will help them oust the conservative Ahmadinejad, whom they accuse of increasing the country's international isolation and compounding its economic difficulties.
Mousavi's campaign appears to have motivated the youth in a country where one-third of the electorate is under 30 and born after the time of the Islamic Revolution in 1979.
"I believe it is a new beginning and I want to take part in it," Parastou Pazhoutan, a 26-year-old Mousavi supporter, said.
"A month ago, I would have said Ahmadinejad was a sure bet,'' Sharif Emam Jomeh, a political analyst, said.
"There was apathy especially with the youth. But now, until 3am, they are out in numbers and they care... Below the surface, something was boiling."