The leading moderate candidate in Iran's presidential election said some of his representatives had been denied access to polling stations on Friday to monitor the vote, the ILNA news agency reported.

Mirhossein Mousavi, a former prime minister who is challenging President Mahmoud Ahmadinejad in the election, called on the authorities to resolve the issue quickly.

He also said text messaging, which candidates have used to reach young voters during a fiercely-contested election campaign, had been closed down and that this was against the law, the ISNA news agency reported.

Ahmadinejad's opponents, who also include liberal cleric Mehdi Karoubi and former Revolutionary Guard leader Mohsen Rezaie, have urged the Interior Ministry and Supreme Leader Ayatollah Ali Khamenei to ensure there is no vote rigging.

Presently they have prevented some of our representatives from being present at polling stations and they do not let us monitor (the vote), ILNA quoted Mousavi as saying. We expect that officials would solve this problem as soon as possible.

Ahmadinejad has ruled out any possibility of fraud and the Interior Ministry has said that candidates' representatives would be allowed to witness the vote at each of the 45,000 polling stations across the country.

Mobile phone users were bombarded with text messages mainly urging support for Mousavi, in the run-up to Friday's vote, but Mousavi said the service had been closed down. Tehran residents were unable to send or receive messages on Friday.

They have shut down SMS. This is against the law. We should not be afraid of the free circulation of information, Mousavi was quoted as saying by ISNA. We ask officials to respect regulations.

Analysts said young, urban pro-reform voters -- many of whom stayed away when Ahmadinejad won a surprise victory in 2005 -- could prove crucial for the outcome as more than 60 percent of Iranians are under the age of 30.

An official of the state telecommunications company confirmed there had been disruptions to the service starting on Thursday evening and that it was being investigated, ISNA said.

During the campaign, backers of Ahmadinejad complained about the sometimes rude jokes aimed at their leader via text messages and Tehran prosecutor's office was reported to have said it would crack down on messages offending candidates.

Popular networking and content-sharing site Facebook was shut off on May 23, joining political and human rights websites which had already been blocked. The Facebook ban was lifted on May 26, following strong criticism from moderate candidates.

More than 150,000 Iranians are Facebook members, and young voters make up a huge bloc, which helped former reformist president Mohammad Khatami win elections in 1997 and 2001.

(Reporting by Zahra Hosseinian; writing by Fredrik Dahl; Editing by Richard Balmforth)

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