Angry protesters pelted a police station and torched an armored vehicle overnight following the death of a bus driver who fought a police officer. Security officials said Saturday they were investigating the case, which reignited long-simmering anger toward Egypt's security forces.

Details of Mohammed Nasr's death were murky. Protesters attacked the east Cairo police station where he had been taken Thursday after a confrontation with a police officer in a busy square. Some witnesses told a lawyer investigating the case that Nasr had been beaten at the station.

Widespread police abuse was one of the driving forces behind the mass protests that erupted in January and forced out President Hosni Mubarak.

Security forces were accused of using excessive force during the uprising. The country's security chief and police officials face trials for the deadly shooting of protesters. At least 846 people were killed during the protests.

Monday marks the anniversary of the death of 28-year old businessman Khaled Said, who was badly beaten by two police agents. Hundreds of thousands joined a Facebook page named after Said that became one of the main tools activists used to mobilize protesters.

The interior ministry has been struggling to restore security and confidence in Egypt's 500,000-strong security forces, which pulled out of Cairo and other major cities three days into the uprising that began Jan. 25. A notorious security agency accused of being behind much of the police abuse was dissolved, but critics say a culture of torture and impunity is still in place.

Outside the Cairo police station late Friday, protesters shouted: "The Interior Ministry has not changed! It is full of thugs!"

An interior ministry official said Nasr slapped a police officer who asked him to clear out of a busy Cairo square.

After being taken to the station, Nasr felt sick and was taken to a hospital where he died, the official said, speaking on condition of anonymity because the investigation was ongoing.

However, a hospital employee who refused to give his name said Nasr left the hospital alive.

Mohammed Abdel-Aziz, a lawyer who specializes in torture cases and is investigating Nasr's death, said he received contradictory statements about it from witnesses. Some witnesses said Nasr was beaten at the police station, Abdel-Aziz said.

Under Mubarak's regime, deaths of detainees in police custody were not uncommon, and indefinite detention and torture were widespread.

"The tension between the people and the police is still there," Abdel-Aziz said.

On Tuesday, the Interior Ministry ordered an investigation into reports that a detainee was tortured to death while in police custody earlier this week.