Clashes erupted between Egyptian protesters and policemen on Saturday outside a Cairo police station over the death of a young man in custody, security sources said.
The protesters torched 10 cars including three police vehicles, they said.
Nine people were injured and 40 people arrested in the overnight clashes in Cairo's Moqattam district which saw some protesters throw petrol bombs as police responded with tear gas and birdshot, they said.
The security sources said a young man nicknamed Afroto, who was arrested on Friday for alleged drug trafficking, died after a violent brawl with other detainees.
But protesters accused the police of being responsible for his death. They set fire to tyres and cars near the police station, leading the fire brigade to intervene.
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Calm returned to Moqattam later in the morning after Cairo's security chief pledged an investigation into the detainee's death, promising not to obscure any police involvement.
The prosecutor examined the body and ordered an autopsy.
Egypt has tried and sentenced several policemen for violent deaths in detention in recent years.
Rights groups have repeatedly denounced alleged torture and deaths in detention.
Egypt's interior ministry has said it does not condone torture but said there have been "individual" cases of abuses.
Police abuses fuelled the 2011 uprising that toppled dictator Hosni Mubarak and ushered in years of political instability.
