In the epicenter of the Arab Spring, opponents and supporters of Egypt's Muslim Brotherhood are clashing in Tahrir Square today, in the first outbreak of factional violence since Islamist President Mohamed Morsi took office. 

Reuters' Yasmine Saleh and Marwa Awad report that 41 have been injured so far in the violence as Islamists and Liberals "threw stones, bottles and petrol bombs, and some fought hand-to-hand." The two sides are at each other's throats over the drafting of the new constitution, which must be completed before a new parliament is elected. A video on YouTube appears to show the protesters clashing in Tahrir Square, though we can't confirm its authenticity:

Leftist groups scheduled today's protest to demand accountability from Morsi after 100 days in office. (They want more representation on the panel that's drafting the constitution.) But the Brotherhood wasn't about to cede the stage and called for a separate rally, which brought in busloads of demonstrators. The Associated Press's Maggie Michael and Aya Batrawy describe how the fighting broke out:

The secular camp accused the group of holding the rally to distract from their anti-Morsi protest. The violence erupted when Brotherhood supporters stormed the activists' stage, angered by chants they perceived as insults to the president. The Islamist backers smashed loudspeakers and tore the wooden stage down, witnesses said.

A melee ensued as the supporters of each camp moved in. Some tore up chunks of concrete and paving stones to hurl, others hit each other with sticks. Gunshots were heard. Two empty buses belonging to the Brotherhood parked near the square were set on fire behind the Egyptian Museum, the repository of Egypt's pharaonic antiquities. No police were on hand in the area during the violence.

Clearly, casting off the yoke of decades of autocracy is not easy. A Reuters photographer captured some compelling images:

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