FACTBOX: Iran's presidential election process

(Reuters) - Iranians vote on Friday in a presidential election in which hardline incumbent Mahmoud Ahmadinejad is seeking a second term against challengers who criticize his handling of the economy and the nuclear issue.

Supreme Leader Ayatollah Ali Khamenei holds ultimate authority in Iran, but the presidential vote will influence the image and policies of the Islamic Republic, now weighing its response to U.S. President Barack Obama's diplomatic overtures.

Following are details of the election and the voting process:

* The four candidates cleared by the Guardian Council are President Mahmoud Ahmadinejad, backed by conservatives; former Prime Minister Mirhossein Mousavi, a moderate; former Parliament Speaker Mehdi Karoubi, a reformist cleric; and Mohsen Rezaie, a former commander of Iran's Revolutionary Guards.

* The Guardian Council, a panel of six senior clerics and six Islamic jurists which vets aspiring candidates, disqualified the rest of the 475 hopefuls who registered, including 42 women.

* To qualify, a candidate must be of Iranian origin and an Iranian citizen, be considered a distinguished political or religious figure and have an unblemished record of piety and fidelity to the Islamic Republic. The Guardian Council bars women from standing although some top clerics and human rights lawyers argue that the constitution does not exclude them.

* All Iranians aged over 18 can vote, which means 46 million of Iran's more than 70 million people are eligible. Campaigning began on May 20 and continues until 0430 GMT on June 11.

* The presidential term is four years. Incumbents can stand for a second term but must then step down for at least one term.

* If no candidate wins at least 50 percent plus one vote of all ballots cast, including blank ones, a run-off round between the top two candidates is held on the first Friday after the election result is declared. A run-off vote took place in the 2005 presidential election, the first since 1979.

(Writing by Parisa Hafezi; editing by Alistair Lyon)