One of the major opposition voices in Iran, Grand Ayatollah Montazeri, died last night of natural causes according to his son. He was 87.

At one time, he was the designated successor to Khomeini - an insider's insider with the ruling clerics. But after strenuously objecting to the massive number of executions and crackdown on a free press undertaken by Khomeini back in the late 80's, he was stripped of his power and put under house arrest.

He repeatedly clashed with the ruling authorities over the years, including once calling out Khamenei for his lack of knowledge of the Koran - in effect, challenging his credentials to be supreme leader. That resulted in the closing of his school, the arrest of some of his students, and 6 years of house arrest.

But it was recently that Montazeri came to the fore as perhaps the most respected - and one of the most beloved - of opposition leaders as this BBC story shows:

Hoseyn Ali Montazeri was a moving spirit in the 1979 revolution which created Iran's Islamic state, and was at one stage set to become its leader.

One of Shia Islam's most respected figures, he was also a leading critic of President Mahmoud Ahmadinejad.

The BBC's Jon Leyne says the death comes at a crucial time in a standoff between the government and opposition.

Iran's rulers will now fear the opposition may attempt a big turnout for his funeral on Monday and other ceremonies marking his death, especially in the run-up to the Shia Muslim festival of Ashura on 27 December, our correspondent says.

Mourners gather

Large crowds have gathered outside Montazeri's home in the holy city of Qom, following his death on Saturday evening.

He will be laid to rest at the shrine of Hazrate Masoumeh, one of the most revered female saints in Shia Islam, his office told AFP news agency.

Thousands of people from Isfahan, Najafabad, Shiraz and other cities are on their way to Qom to attend Monday's ceremony, according to one report.

The cleric's son told the BBC that his father had died of natural causes.

Would the authorities dare crackdown on dissidents who attend the funeral?

To disrupt the funeral of such a holy man - second only to Khomeini in the eyes even of many regime supporters - would be suicide. But Khamenei and Ahmadinejad have already warned protestors what would happen if they turn out in the streets so it will no doubt be a tense time in Qom tomorrow.