TEHRAN, Iran - The spiritual father of Iran's reform movement died yesterday at the age of 87, prompting thousands of his followers to immediately head to the holy city where the dissident cleric is to be buried.

A huge display of mourning for Grand Ayatollah Hossein Ali Montazeri could transform his funeral today into another display of power by the opposition, whose activists have for months defied a brutal crackdown, a mass trial and abuses in detention to denounce the country's hard-line clerical rulers.

Opposition leaders called for people to turn out for a day of mourning.

Police reinforcements were also called out in Qom, the religious center south of the capital where today's commemorations will take place, an opposition Web site reported.

And a prominent government critic who was one of Montazeri's students was arrested on his way to the city, a human rights group said.

Authorities also banned foreign journalists from traveling there to cover the events.

Montazeri was a key figure in the 1979 Islamic Revolution who later accused his fellow clerical leaders of imposing dictatorship in the name of Islam.

His criticism persisted after June's disputed presidential election ignited a new wave of anti-government protest.

In particular, he opened the door to direct condemnation of the ruling clerics, a bold step that energized Iran's young activists. In August, he decried the "despotic treatment" of protesters at the hands of the ruling theocracy. A month later, he accused the regime of committing "crimes . . . in name of Islam."

Despite his stature, Montazeri's death is not likely to have a profound impact on the opposition movement, which has moved past allegations of vote rigging to assert that the entire ruling system has been corrupted, said Abdulkhaleq Abdulla, a professor of political science at the United Arab Emirates University.

"I think the current opposition movement has gone way beyond and above what Montazeri was standing for," Abdulla said.

But his strong denunciations against Supreme Leader Ayatollah Ali Khamenei helped break the taboos against such criticism and pushed the protesters into bolder acts of defiance.

In demonstrations earlier this month, students shouted "Death to the dictator!" and burned pictures of Khamenei.

Khamenei issued a statement of condolence yesterday that contained a mixed message.

He praised Montazeri as an outstanding jurist, but added that he hoped God would forgive him for what he called Montazeri's "crucial test," a reference to his falling out two decades ago with the Islamic Republic's founder, Ayatollah Ruhollah Khomeini. The split between those two led to a long government campaign to marginalize Montazeri that included five years of house arrest in which security agents were posted just outside his front door.

Montazeri's grandson, Nasser Montazeri, said he died in his sleep overnight. The Web site of Iranian state television quoted doctors as saying Montazeri had suffered from asthma and arteriosclerosis, a disease that thickens and hardens arteries.

Authorities in Qom faced a difficult choice over whether to try to prevent an outpouring at the funeral that could turn into another opposition street protest.

Doing that risks backlash from an influential group of clerics based there who are among the current leadership's critics.