Iranian opposition sources said on Sunday that thousands of mourners converged on the city of Qom for the funeral on Monday of Iran's top dissident cleric Grand Ayatollah Hossein Ali Montazeri.

Montazeri, 87, a fierce critic of President Mahmoud Ahmadinejad, died of an illness on Saturday night and is to be buried in Qom, said his office in the central city where he was based.

"He was diabetic and had been using insulin for years. He had also some lung problems and asthma. In fact he was suffering from several diseases," his doctor told state television.

The grand ayatollah was an inspiration to rights advocates and pro-reform groups and was considered by his followers as the highest living authority of Shiite Islam in Iran.

Key opposition leaders Mir Hossein Mousavi and Mehdi Karroubi called for a public day of mourning on Monday and for their supporters to take part in the funeral, in a joint statement on Mousavi's website, Kaleme.org.

Upon the news of his death, students gathered at Tehran University to recite verses from the Koran.

The opposition website Rahesabz.net said opposition supporters also called a gathering to mourn the cleric at Tehran's Mohseni square on Sunday, adding that "sporadic gatherings" were seen in the capital.

Montazeri is to be buried in the shrine of Masoumeh, a revered Shia figure, in Qom, his office told AFP. Foreign media have been banned from covering the ceremony. 

-AFP/NOW Lebanon