Hamda Saber said he was initially coerced by police into giving a false account of the attack
The US has urged Egypt to investigate cases of police abuse after a man was stripped and beaten by uniformed officers near the presidential palace.
The state department said it was "extremely disturbed" by the dragging of naked Hamada Saber, which was caught on camera, through the streets.
Egypt's culture minister has resigned in the aftermath of the incident.
Protests were continuing late on Monday after the funeral of another activist beaten by police, Mohammed al-Guindi.
Opponents of Islamist President Mohammed Morsi say the death proves the police have not reformed in the two years since authoritarian President Hosni Mubarak was ousted.
"We urge the government of Egypt to thoroughly, credibly and independently investigate all claims of violence and wrongdoing by security officials and demonstrators and to bring perpetrators to justice," said state department spokeswoman Victoria Nuland.
"Accountability is the best way to prevent recurrences of these kinds of incidents."
For his part, Mr Morsi said in a Facebook message that he had asked the public prosecutor to investigate the death of Mohammed al-Guindi.
He emphasised there was "no return to rights abuses of citizens and their freedoms... after the January 25 revolution" - in reference to the popular uprising that toppled Mubarak in February 2011.
On Sunday, Mr Saber said he had been coerced by police into initially giving a false account of his attack, saying that had he blamed them, they would have accused him of carrying petrol bombs at the demonstration.
The 50-year-old painter said police had subsequently apologised to him for any wrongdoing.
State TV reported Culture Minister Mohammed Saber Arab resigned on Monday.
Protesters accuse Mr Morsi of betraying the aims of the 2011 uprising - a claim he denies.
The current unrest began on 24 January in Cairo on the eve of the second anniversary of the revolution and has spread to several cities.
Egyptian army chief Gen Abdul Fattah al-Sisi last week warned that the political crisis could lead to the collapse of the state.