TEHRAN, Iran - The religious leader of Iran's opposition movement, Grand Ayatollah Hossein Ali Montazeri, 87, died yesterday at his home in the Shiite Muslim holy city of Qom.

His death deals a blow to human rights and democracy advocates who considered him their spiritual guide.

Doctors and relatives said he died in his sleep of multiple organ failure.

The death of the ayatollah, who was once designated to lead the Islamic Republic, comes during an already tense Shiite Muslim mourning period, called Muharram, in which millions will take to the streets in the coming nights to mourn the death of a revered Shiite saint.

State media omitted Ayatollah Montazeri's religious titles in brief reports of his death, a sign they were playing down his influence.

Some opposition figures fear that authorities will try to prevent mourning ceremonies for Ayatollah Montazeri, fearing they might turn into antigovernment protests.

Opposition leaders Mir Hossein Mousavi and Mehdi Karroubi declared today a day of public mourning.

"There are many security forces stationed in the streets," said Saeed Montazeri, 41, one of the ayatollah's children, speaking by phone from Qom. "But they are not doing anything for now." In the background, the sounds of a crowd could be heard.

In Najafabad in Isfahan province, where he was born in 1922, demonstrations broke out, according to video clips posted on opposition Web sites.

Authorities denied foreign reporters permission to travel to Qom, 90 miles south of Tehran, and ordered domestic news media to emphasize criticisms of Ayatollah Montazeri by the late Ayatollah Ruhollah Khomeini, leader of Iran's 1979 Islamic revolution.

Iran's supreme leader, Ayatollah Ali Khamenei, issued a statement of condolence, the state-run Islamic Republic News Agency reported. He mentioned Ayatollah Montazeri's services to the revolution, but also touched on the cleric's differences with Khomeini.

After Grand Ayatollah Ali Sistani, an Iranian-born Shiite cleric residing in Najaf, Iraq, Ayatollah Montazeri was considered the highest Shiite authority in the world.

As one of Iran's main revolutionary clerics supporting Khomeini, he helped oust the U.S.-backed Shah Mohammad Reza Pahlavi in 1979 and strongly supported spreading the Islamic revolution.

In 1987, one of his closest assistants was executed for leaking information on a secret weapons deal with the United States in what came to be known as the Iran-contra scandal. The revelation, embarrassing to Iran's leaders, was aimed at sabotaging the deal, which Ayatollah Montazeri and his supporters said was against the revolution.

In a dramatic turn of events in 1989, the ayatollah, who had been designated to succeed Khomeini as Iran's supreme leader, had a public falling out with the leadership for opposing the mass execution of political opponents. As a result, he was placed under house arrest.

There, he kept a low profile teaching in the local religious schools, only to return to the political stage in 1997 with a speech challenging the credentials of Khomeini's successor as supreme leader, Ayatollah Khamenei.

His criticism led to tighter house arrest, which was quietly lifted in 2003.

In July, a month after the presidential election dispute sparked riots in Iranian cities, he issued fatwas, or religious edicts, opposing the government of President Mahmoud Ahmadinejad.

Politicians in the opposition regard him as a hero who defended democratic values.