Former Iranian President Akbar Hashemi Rafsanjani has officially confirmed he will stand for election once more.
Rafsanjani has been president twice before
Regarded as the front-runner in the 17 June poll, Mr Rafsanjani had said he would not run unless supported by supreme leader Ayatollah Ali Khamenei.
Mr Rafsanjani, 70, is seen as a pragmatic conservative, our correspondent in Tehran says.
The current President, Mohammad Khatami, is barred by the constitution from seeking a third consecutive term.
Several unknown candidates were the first to register, including some women - who are not eligible to stand for the post under Iranian law.
Candidate registration began on Tuesday and will continue until Saturday evening.
The presidential hopefuls included a 21-year-old woman, a man dressed like Osama Bin Laden and a former diplomat from the time of the shah.
The candidates all have to be vetted by the Guardian Council, a powerful clerical body that scrutinises whether they are honest people and believers in the values of the Islamic republic.
"Running in the June 17 election has been one of the most difficult decisions in years of my life," Mr Rafsanjani said in a three-page statement.
Candidates with little chance were the first to register
He said Iran was facing "destructive tensions that prevent the development of the country" and threats to "rights and fundamental freedoms - in what many observers said was a clear reference to religious right-wingers.
Mr Rafsanjani is seen as someone open to better ties with the West, but is more socially conservative than the reformists, the BBC's Tehran correspondent Frances Harrison says.
Mr Rafsanjani, who has been president twice from 1989 to 1997, had hesitated to announce that he would definitely be standing.
He has been part of the system for decades, our correspondent says.
She adds that he was said to be wary about being pushed into the role of an opposition president at odds with Iran's powerful supreme leader.
That was the problem of reformist President Mohammed Khatami, who received a huge popular mandate initially but was unable to deliver on his promises because he did not have sufficient power as president.
Ayatollah Khamenei said on Monday that he did not want to hand over the whole country to one political faction, a statement that was interpreted as a signal to Mr Rafsanjani that he should go ahead and stand.