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Around 500 young people from different movements, including the powerful Islamist opposition, had braved rain and cold weather to call for "regime" reforms and putting the corrupt on trial.
They were camped out next to the Interior Circle, or Gamal Abdel Nasser Square, in the capital.
At nightfall, police attempted to disperse the youths and cut off electricity on the square around 11:00 pm (2100 GMT), an AFP journalist witnessed.
"A group of nearly 50 loyalists who were gathered not far from them (the students) took advantage of the power cut to throw rocks at the youths," one of the protesters, Moaz Kassrawi, told AFP.
"Most of those injured suffered head wounds and had to be hospitalised," he said.