MOSCOW, July 17 (RIA Novosti) - Thousands of supporters of Iran's defeated presidential candidate Mir-Hossein Mousavi demonstrated in Tehran on Friday after a powerful cleric delivered a sermon casting doubt on the recent election.

Akbar Hashemi Rafsanjani said his country remains in crisis after the disputed poll, defying Supreme Leader Ayatollah Ali Khamenei, who has upheld the election result and accused external forces of stirring the unrest.

The cleric criticized the authorities' handling of the poll, and said voters' faith in the system must be restored.

"That trust cannot be brought back in a day or a night," he said. "We all have been harmed. Today more than ever we need unity."

Police fired tear gas and used batons to disperse demonstrators, arresting at least 15 people.

Tehran was hit by mass demonstrations over alleged ballot fraud after the landslide reelection of hardline President Mahmoud Ahmadinejad on June 12.

A Russian expert warned on Friday that a new wave of unrest could erupt in Iran over the election.

"This possibility cannot be ruled out," said Azhdar Kurtov, an expert at the Institute of Strategic Studies. "There are doubts over the fairness of the ballot count."

However, he said that protests would not lead to the scrapping of election results and the holding of a repeat election.