Police fired tear gas at protesters in the Nile Delta city of Tanta on Monday after the funeral of an Egyptian activist who slipped into a coma and died after several days in custody, witnesses said.
Mohamed al-Guindi, 28, was arrested in Cairo's Tahrir square after joining protests demanding change on the second anniversary of the country's uprising. The opposition says his death on Monday proves police have not reformed.
The death came as police faced a backlash after riot policemen were filmed beating a dragging a naked man during clashes on Friday outside the presidential palace in Cairo, prompting a minister to resign, according to state media.
The official MENA news agency reported that Culture Minister Mohammed Saber Arab tendered his resignation on Monday over the incident. The state's Al-Ahram newspaper reported on its website that Saber resigned over police abuse.
In Tanta, witnesses told AFP that police fired tear gas on a march that approached the provincial headquarters, following Guindi's burial in the city.
Guindi's lawyers say that after his arrest he was taken to a police camp in Al-Gabal al-Ahmar in Cairo where he was subjected to torture, before being sent several days later to Cairo's Al-Helal hospital.
According to the health ministry, Guindi arrived at the hospital unconscious and suffering from internal bleeding. The Popular Current, to which Guindi belonged, said the activist died "as a result of torture."
Guindi's death and the footage of police beating a protester have confronted Islamist President Mohamed Morsi with uncomfortable parallels with Hosni Mubarak's regime, deposed in 2011 after an uprising partly aimed at police abuses.
The presidency said in a Facebook statement that it had asked the public prosecutor to probe Guindi's death, stressing there is "no return to rights abuses of citizens and their freedoms... after the January 25 revolution."
Two years ago, Egyptians poured into the streets demanding a complete overhaul of the interior ministry, whose police officers were accused of systematic abuse under a culture of impunity.
But images of Guindi's face bruised and battered on a hospital bed and the footage of Ahmed Saber's beating show how little has changed, rights groups say.
In a report published on the anniversary of the uprising, the Egyptian Initiative for Personal Rights said police violence was still rampant.
"The Egyptian police continue to systematically deploy violence and torture, and at times even kill," EIPR said.
"There has been no thorough change, or even cosmetic improvement, in the police apparatus, whether related to its administrative structure, decision-making, oversight of police work or the reform and removal of leaders and personnel responsible for torture and killing," it said.
The group said it investigated dozens of cases of torture in recent months, noting "the consistent use of physical and psychological violence against detainees, which in many cases led to death."
Within hours of the news of Guindi's death, furious activists took to social networking sites condemning police brutality and calling for change.
Guindi "is not the first to die (of torture) under Morsi. But he is a political activist, that's why you know his name," tweeted Hossam Bahgat, EIPR's director.
"Youth still tortured & killed in quest for human dignity. Regime oblivious that violence begets violence & brutality is sure to backfire," said leading dissident and former UN nuclear watchdog chief Mohamed ElBaradei.
"Behind every martyr who dies is a corrupt police force," said rights activist Gamal Eid.
"We will not get out of this cycle of violence except with the success of our revolution, which primarily called for the end of the police state and the oppression of the interior ministry," said Nirvana Shawky of the Egypt Freedom party.
Hundreds turned out in Tahrir Square for funeral prayers for Guindi and another activist who was shot dead in protests over the weekend, including Popular Current chief Hamdeen Sabbahi.
"Either we get justice for them, or we die like them," the mourners chanted.
Tributes have poured in for Guindi on Twitter and Facebook, with activists dubbing him the "new" Khaled Said, an Egyptian man who was beaten to death by police in 2010 and who became a symbol of the fight against police brutality.