Egyptian children are subject to arbitrary arrest by police, who abuse them physically and sexually and extort money from them, Human Rights Watch claims.
A report by the international organisation says children are jailed alongside adults, who also maltreat them.
Clarisa Bencomo, who helped research the report, said street children are the most vulnerable - and are often arrested without cause.
"Instead of protecting children, the police abuse them and steal whatever money they have," she said.
He said, 'If you are really a girl, take off your clothes so we can examine you.' Then he grabbed my breasts, but I hit him
"Children are particularly vulnerable to sexual abuse, both in and out of custody," she said.
The report, entitled "Charged with Being Children: Egyptian Police Abuse of Children in Need of Protection", was issued in English and Arabic.
It quotes children as young as 11 who have been arrested.
One 16-year-old girl, who the report calls "Warda N" to protect her identity, described being sexually harassed by police.
"The guard here says 'You are a woman.' He keeps saying that to me. I keep saying, 'No, I'm a girl [that is, a virgin].'
"Yesterday he said, 'If you are really a girl, take off your clothes so we can examine you.' Then he grabbed my breasts, but I hit him," she told the researchers.
An eleven-year-old boy whom the report calls "Yahiya H" said he was handcuffed and put in a transport with adult prisoners.
"There were 50 or 60 people in the transport vehicle - adults and kids. I had handcuffs on and the adults did too. I couldn't breathe. I thought I was going to die," he said.
The prosecutor says 'You stole something.' I say, 'I didn't steal anything.' Then he says, 'OK, begging.'
"I was screaming, but no one did anything. They didn't open the door until we arrived. There were small kids crying, but no one did anything for them," he said.
"Khaled M", also 11, said officials level arbitrary charges against street children.
"The prosecutor says 'You stole something.' I say, 'I didn't steal anything.' Then he says, 'OK, begging.'", the report quotes him as saying.
Human Rights Watch says police use the country's Child Law to "clear the streets of children, extort money and information, force children to move on to other neighbourhoods, and bring children in for questioning in the absence of evidence of criminal wrongdoing".
Ms Bencomo said that children come out of the criminal justice system in worse shape than when they went in.
"Most of these children are back on the street within a week, in even worse shape than before. Instead of protecting children, the police abuse them and steal whatever money they have," she said.
Call for action
The organisation called on Egypt to "immediately end its policies of arresting children ... and of routinely detaining children in police lockups".
It urged the government to protect children in accordance with the Convention on the Rights of the Child.