Iranian President Hassan Rouhani, meanwhile, was selected to run again
Former Iranian leader Mahmoud Ahmadinejad, who ruled Iran from 2005 to 2013, was barred from running in next month's election, while President Hassan Rouhani was among six candidates approved by Iran's conservative-controlled Guardian Council, state media reported on Thursday.
Ahmadinejad shocked everyone by registering as a candidate last week against the advice of Supreme Leader Ayatollah Ali Khamenei - a move that many described as political suicide.
Khamenei had advised Ahmadinejad not to run, and his attempt to become a candidate was widely seen as a public snub to the Supreme Leader, which is nearly unheard of in the Islamic Republic.
The disqualification of Ahmadinejad, a two-term president, draws attention to the criteria that the Guardian Council, the governmental body which vets candidates, uses in the selection process.
Khamenei appoints half of the members of the Guardian Council, and by disqualifying Ahmadinejad, the body runs the risk of being seen as a rubber stamp for the supreme leader, who is the highest authority in the country.
More than 1,600 candidates registered to run in the 19 May election, but the Guardian Council usually selects about half a dozen.
More than 130 women registered, but none has ever been allowed to stand.
The other candidates selected were hardliners Ebrahim Raisi and Mostafa Mirsalim, Tehran Mayor Mohammad Bagher Ghalibaf, moderate Mostafa Hashemitaba and Rouhani's ally and vice-president Eshaq Jahangiri.
"In Iran, it's not only an election, it's also a selection," said Clement Therme, Iran research fellow for the International Institute for Strategic Studies.
Campaign starts now
Although campaigning was not due to start until 28 April, the Guardian Council announced that it could begin immediately.
Rouhani, a politically moderate cleric, has won praise since his landslide win in 2013 for taming inflation and reaching a groundbreaking nuclear deal with world powers that ended many sanctions.
But disappointment over Iran's continued economic stagnation is palpable in the streets, creating an opening for conservative opponents.
'His administration has a discourse of social justice but they are ultimately neoliberal, and this has provoked disappointment'
- Clement Therme, International Institute for Strategic Studies
Unemployment is stuck at 12 percent, the promised billions in foreign investment have not materialised and Rouhani has failed to release political prisoners, including reformist leaders under house arrest for their part in 2009 protests.
"The problem has been the nature of Rouhani's economic agenda. His administration has a discourse of social justice but they are ultimately neoliberal, and this has provoked disappointment," said Therme.
The aggressive stance of US President Donald Trump, who has slapped new sanctions on Iran and threatened to tear up the nuclear deal, has bolstered conservative claims that Rouhani's outreach to the West has been misguided.
The conservatives are divided, but Raisi has garnered considerable momentum.
The 56-year-old judge, who currently runs the powerful charity-cum-business-empire Astan Qods Razavi, has emphasised his concern for the poor and is seen as a close ally of - and possible successor to - Khamenei.
No live debates
Tehran mayor Ghalibaf, who came second to Rouhani in 2013, is also considered a front-runner.
The 55-year-old is a war veteran, former Revolutionary Guard commander and police chief, and considered a staunch if pragmatic conservative.
He has focused his early comments on the economy, saying he would create five million jobs and more than double Iran's revenues, although such promises have been ridiculed as wildly unrealistic by reformist opponents.
Mirsalim, 71, was a culture minister in the early 1990s, known for dramatically increasing censorship. He belongs to one of the older conservative parties, the Islamic Coalition, that has been somewhat sidelined by new hardline groupings in recent years.
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Hashemitaba is a veteran politician in his 60s who has served variously as industry minister, vice president and head of Iran's National Olympic Committee through several administrations since the revolution, though he has figured only marginally in local coverage of the election so far.
Rouhani was hit by the loss of a key backer in recent months: heavyweight former president Akbar Hashemi Rafsanjani, who passed away in January, played a central role in his surprising landslide victory in 2013.
Rafsanjani's brother, Mohammad Hashemi, was also among those disqualified on Thursday.
But crucially, Rouhani retains the unified support of moderates and reformists, who still see him as the best hope for change within the strict parameters of Iran's Islamic system.
Jahangiri said he was only standing in the election to support Rouhani during the campaign and in TV debates, saying last week that he stood "side-by-side" with the president.
Iran's election commission ruled on Thursday that there would be no live debates in the run-up to the election - a feature that has been hugely popular since they were introduced in 2009. Debates will instead be pre-recorded.