After eight days of protests that killed nearly 60 people, a video of one demonstrator stripped, dragged across the ground and beaten has fired Egyptians to a new level of outrage.
Hamada Saber, a middle-aged man, lay in a police hospital this morning, after he was shown on television naked, covered in soot and thrashed by half a dozen policemen who had pulled him to an armoured vehicle near the presidential palace.
President Mohamed Mursi's office promised an investigation of the incident, which followed the deadliest wave of bloodshed of his rule.
His opponents say it proves that he has chosen to order a brutal crackdown like that carried out by Hosni Mubarak against the uprising that toppled him in 2011.
"Mursi has been stripped bare and has lost his legitimacy. Done," tweeted Ahmed Maher, founder of the April 6 youth movement that helped launch the anti-Mubarak protests.
Another protester was shot dead yesterday and more than 100 were injured, many seriously, after running battles between police and demonstrators who attacked the palace with petrol bombs.
That unrest followed eight days of violence that saw dozens of protesters shot dead in the Suez Canal city of Port Said
Mr Mursi responded by declaring a curfew and state of emergency there and in two other cities.
But none of the bloodshed - which the authorities have blamed on the need for police to control violent crowds - has quite resonated like the images of police abusing a man at their feet - clearly helpless, prone and no possible threat.
The incident was an unmistakable reminder of the beating of a woman by riot police on Tahrir Square in December 2011.
Images of her being dragged and stomped on - her black abaya cloak torn open to reveal her naked torso and blue bra - became a rallying symbol for the revolution and undermined the interim military rulers who held power between Mr Mubarak's fall and Mr Mursi's rise.
The rise of Mr Mursi - the first freely elected leader in Egypt's 5,000-year history - is probably the single most important change achieved by two years of revolts across the Arab world.
But seven months since taking office, he has failed to unite Egyptians. Street unrest and political instability threaten to render the most populous Arab state ungovernable.
The latest round of violence was triggered by the second anniversary of the uprising against Mubarak and death sentences handed down last week in Port Said over a soccer stadium riot.
Mr Mursi has had little opportunity to reform the police and security forces he inherited from Mubarak and the military men.
But the police action against protests this time has been far deadlier than it was even a few months ago, when bigger crowds demonstrated against a new constitution.
That suggests to opponents that Mr Mursi has ordered a tougher response.
The liberal, leftist and secularist opposition accuses Mr Mursi of betraying the revolution that toppled Mubarak by concentrating too much power in his own hands and those of his Muslim Brotherhood, a formerly underground Islamist movement.
Mr Mursi and the Brotherhood accuse the opposition of stoking street unrest to further their demands for a national unity government as a way to retake power they lost at the ballot box.
In announcing an investigation into the beating of Saber, Mr Mursi's office made clear he was still pointing the blame at the political opponents who have encouraged protests.