A video of a demonstrator stripped naked, dragged across the ground and beaten with truncheons by riot police has generated a new level of outrage in Egypt after eight days of protests in which nearly 60 people have died.
Hamada Saber was in a police hospital on Saturday after he was pulled into an armoured vehicle near the presidential palace.
President Mohamed Mursi's office has promised an investigation.
The incident was an reminder of the beating of a woman by riot police on Tahrir Square in December 2011. Images of this became a rallying symbol for the revolution.
His opponents say it proves he has chosen to order a brutal crackdown like that carried out by former President Hosni Mubarak against the uprising that toppled him in 2011.
Another protester was shot dead and more than 100 were injured, many seriously, after running battles between police and demonstrators throwing petrol bombs outside the presidential palace in Cairo.
The latest round of trouble was triggered by the second anniversary of the uprising against Mubarak and 21 death sentences handed down last week in Port Said over a soccer stadium riot in February last year, in which 74 people were killed.
Mr Mursi and the Muslim Brotherhood accuse the opposition of stoking street unrest as a way to retake power they lost at the ballot box.
