Today is the Shiite Muslim holiday of Ashura. In Iran, it has meant bloody clashes between protestors and regime thugs:

Iranian police opened fire on protesters in Tehran on Sunday, killing at least four people, including a nephew of the opposition leader Mir Hussein Moussavi, as vast crowds of demonstrators flooded the streets of cities across Iran and fiercely fought security forces, according to witnesses and opposition Web sites.

The protests, taking place on the holiday marking the death of Shiite Islam’s holiest martyr, were the bloodiest – and among the largest – since the uprisings that followed Iran’s disputed presidential election last June, with hundreds of thousands of people thronging Tehran alone, witnesses said. There were reports of hundreds of injured people and numerous arrests.

[…]

The protesters deliberately blended their opposition message with the day’s religious meaning, alternating anti-government slogans with ancient cries of mourning for the prophet Mohammed’s grandson, Hussein, the 7th-century saint whose death in battle is commemorated on the Ashura holiday.

“This is the month of blood, Yazid will fall!” the protesters shouted, equating Iran’s supreme leader, Ayatollah Ali Khamenei, with Yazid, the ruler who ordered Hussein’s killing.

Just as they did during protests earlier this year, Iranians on Twitter have done a good job disseminating images and news (and, occassionally, unconfirmed rumors). This account has been especially active today.