At least 100 people were killed early Saturday when riot police fired on protesters supporting deposed Egyptian President Mohammed Morsi in Cairo, activists said.

The Anti-Coup Alliance, an umbrella coalition of Morsi supporters, also said at least 5,000 people were injured near the clash in the Nasr City neighborhood of the capital.

Many of the bodies of those killed were still at a field hospital near the pro-Morsi protest site and were not officially registered with health officials. The official death toll was based only on bodies tallied at government morgues, so that figure was only at 21.  [as of 9:40 am PDT that figure has been upgraded to 38 killed]

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Morsi, who was elected by a narrow margin in June 2012 in the country’s first democratic vote, has been hidden from the public since he was ejected from power. On Friday, Mena news agency reported he would be detained for 15 days while a judge investigated allegations against him. This is his first formal detention since he was ousted, Al Jazeera reported.

According to Reuters, the probe stems from charges that he conspired with Palestinian Islamist group Hamas to escape jail during the 2011 uprising against former President Hosni Mubarak, killing prisoners and officers, kidnapping soldiers, and lighting buildings on fire.