More than 90 Iranians applied on Tuesday - the first day of registration - with the country's Interior Ministry scheduled to keep the registration offices open for five days.
The likely frontrunner, former president Hashemi Rafsanjani, declared on Tuesday definitely for the first time that he will run.
Aljazeera said he cited several reasons for his decision, including his belief that a new Middle East could not be shaped without Iran's participitation and that Iran's isolation was unacceptable.
Opinion polls suggest Rafsanjani is likely to win despite the uncertainty surrounding his candidacy.
"Running in the 17 June election has been one of the most difficult decisions in years of my life," Rafsanjani said in a public statement.
Third term barred
Outgoing President Khatami came to power in a landslide in 1997 but his attempts to bring political and social reforms were stifled by hardline clerics led by supreme leader Ayatollah Ali Khamenei.
Khatami is barred by law from seeking a third term.
Big names were absent from the registration process at the Interior Ministry building. Instead, most applicants were members of the public looking to try their hand - including
Touran Jamili was one of the 90
Iranians who applied on Tuesday
an 18-year old girl, a guard at a tile factory and a man who shouted "Death to America!" as he filled in the registration form.
Most knew they had little chance of being allowed to run, hoping instead to make a political statement or just get the attention of friends.
The hardline Guardian Council, a constitutional watchdog that supervises the elections, will have a week to vet the applicants. The council has already said it will not allow women to run.
Major candidates usually wait until the last days of the registration period, which ends on Saturday evening.
Grip on power
The ruling clerics see the forthcoming vote as a chance to consolidate their grip on power, and a number of close Khamenei loyalists are expected to run.
Among them are the former commander of the elite Revolutionary Guards, Mohsen Rezaei; the top police commander Mohammad Bagher Ghalibaf; Tehran mayor Mahmoud Ahmadinejad; the former head of state radio and television Ali Larijani; former foreign minister Ali Akbar Velayati; and lawmaker Ahmad Tavakoli.
Ali Akbar Nassiri Khanasari chants
anti-US slogans while registering
Their strongest rival will not be a reformer - but Rafsanjani, a savvy pragmatist who has frequently gone back and forth between the hardline and more moderate camps.
He has in the past sought to make contacts with the United States, a stance shunned by hardliners.
Iran's largest reform party, the Islamic Iran Participation Front, has chosen former cabinet minister Mostafa Moin, a close Khatami ally, as its candidate. Other reformists support former parliamentary speaker Mahdi Karroubi.