The situation in Iran is still tense as demonstrators clash with government forces who reportedly kill the nephew of Mir Hussein Mousavi:

TEHRAN -- Security forces opened fire at crowds demonstrating against the government in the capital on Sunday, killing at least four people in the heaviest clashes in months, witnesses and websites linked to the opposition said.

The nephew of the opposition's political leader, Mir Hussein Mousavi, was allegedly killed, according to Parlemannews, a Website managed by a faction in the Iranian parliament which supports the opposition. "Ali Mousavi, 32, was shot in the heart at the Enghelab square. He became a martyr," the Rah-e Sabz Website reported.

Fierce battles erupted as tens of thousands of demonstrators tried to gather on a main Tehran street, with people setting up roadblocks and throwing stones at members Special Forces, under the command of the Revolutionary Guard Corp. They in turn threw dozens of teargas and stun grenades, but failed in pushing back crowds, who shouted slogans against the government, witnesses reported.

Fights were also reported in the cities of Isfahan and Najafabad in central Iran.

The protests coincided with Ahsura, one of the most religious festivals for Shiite Muslims. The slogans were mainly aimed at the top leaders of the Islamic republic, a further sign that the opposition movement against President Mahmoud Ahmadinejad's disputed June election victory is turning against the leadership of the country.

At the Yadegar overpass, protesters shouted slogans such as "Death to the dictator" and "long live Mousavi." They fought running battles with security forces until a car filled with members of the paramilitary Basiji brigade drove at high speed though the make shift barriers of stones and sandbags that the protesters had erected.

About a dozen members of the Revolutionary Guards fired paintball bullets, teargas and stun grenades. When reinforcements arrived, they managed to push back the hundreds of protesters gathered at the crossing.

Similar scenes could be seen at several crossings of the central Azadi and Enghelab streets, witnesses reported. Large clouds of black bellowing smoke rose up as people honked their cars in protests.

"This is a month of blood. The dictator will fall," people shouted, referring to the mourning month of Moharram. Young men erected a flag symbolizing the struggle of the Shiite's third Imam Hussein, whose death was commemorated Sunday.

Update from Juan Cole:

The Mousavis are putative descendants of the Prophet Muhammad, a sort of caste in Muslim societies called 'sayyid' or 'sharif.'

In fact, in the Constitutional Revolution of 1905-1911, one of the complaints of the crowd was that the Qajar monarchy had had sayyids beaten. So if beating a scion of the House of the Prophet can help spark a revolution, what about shooting one? And, oppositional film maker Mohsen Makhmalbaf maintains that Mousavi was killed by a death squad that came for him in a van rather than just falling victim to random police fire.

Killing a sayyid is a blot on any Iranian government. Doing so on Ashura, the day of mourning for the martyred grandson of the Prophet, Imam Husayn, borders on insanity.