Hossein Ali Montazeri was always deeply inconvenient to the Iranian regime — one of the most senior clerics in the country, he was unafraid to denounce the bloody tyranny wrought by the Islamic revolution he had supported. Until 1989, when he objected to the execution of thousands of government opponents and thus had a falling out with Ruhollah Khomenei, he was seen as next in line to be Supreme Leader (the job, of course, went to Ali Khamenei, despite his being an Ayatollah of the non-“Grand” variety and therefore outranked by Montazeri). Montazeri spent the next two decades criticizing the regime, sometimes from under house arrest, and was a supporter of the opposition movement that drew protestors to the streets of Iran in the wake of the obviously fraudulent election results earlier this year. And because of his clerical credentials, the regime could never bring themselves to silence him.

He died in his sleep last night at the age of 87, reportedly of heart failure. His supporters’ demonstrations of mourning have the potential to spark a major confrontation:

Senior opposition leaders, including the former presidential candidates Mir Hossein Moussavi and Mehdi Karoubi, immediately began urging supporters to flock to the holy city of Qum for his funeral on Monday. And the Iranian authorities were clearly bracing for a showdown there: there were reports Sunday of riot police forces already gathering in the city, and Iranian news sites said the government was planning to close the main highway between Tehran and Qum.

In Tehran, hundreds of protesters marched at Tehran University and at the University of Science and Industrys, chanting, “Montazeri is alive!” And an opposition Web site, Peykeiran, reported that demonstrators set fire to two buses in the ayatollah’s hometown, Najafabad, and that riot police were opposing them there.

Large opposition protests had also been planned on the religious holiday of Ashura, on Dec. 27. That will coincide with the seventh day after Ayatollah Montazeri’s death, an important marker in Shiite mourning ritual.

Here’s a YouTube video of protesters in Tehran.