Iranian media say more than 600 candidates have registered to run in next month's presidential election.

At least 638 have registered in the first three days of the process through Thursday, more than twice the number that had registered during the same period in 2013. Registration closes on Saturday.

More people tend to run when a moderate is in office because the political sphere is more open. More than 1,000 people registered in 2005, under reformist President Mohammad Khatami.

The Guardian Council, a clerical body that oversees elections, is expected to bar most candidates and will announce an approved list by April 27.

The most high-profile candidate to register thus far is former hard-line President Mahmoud Ahmadinejad. President Hassan Rouhani, a moderate, is expected to seek re-election.