Iran's most senior dissident cleric, Grand Ayatollah Hossein Ali Montazeri, has died at the age of 87, providing a fresh catalyst for renewed demonstrations against the country's hardline leadership.

Supporters of Ayatollah Montazeri, an architect of the 1979 Islamic revolution that toppled the US-backed shah, were yesterday flocking to the Shi'ite holy city of Qom for the cleric's funeral today, the moderate Parlemannews website said.

Opposition leaders Mirhossein Mousavi and Mehdi Karoubi urged supporters to attend the funeral, declaring today a day of national mourning, the reformist Jaras website said.

Riot police were already on the streets in parts of Qom, where Ayatollah Montazeri lived and died, the reformist Tagheer website said. Demonstrations in Qom, the seat of Shi'ite learning in Iran, would embarrass Iran's hardline rulers, particularly if large numbers of Islamic seminary students were to join in.

Ayatollah Montazeri's death from a heart attack, reported by official media yesterday, coincides with tension rising once again in the Islamic Republic, six months after the presidential poll plunged the major oil producer into political crisis.

"My grandfather died in his sleep last night. People and friends are coming to express their condolences," Naser Montazeri said from Qom, 125 kilometres south of Tehran.

Hundreds of Montazeri supporters took to the streets in his home town of Najafabad, both mourning his loss and chanting slogans, video posted on the internet showed. Shops in the traditionally moderate town had their shutters down and cloaked in black cloth pinned with pictures of the late cleric.