Another child of the same age when arrested in January, Aser Mohamed, "was beaten, given electric shocks all over his body and suspended from his limbs in order to extract a false 'confession',"
Egyptian police have been implicated in an "unprecedented spike" in enforced disappearances since early 2015 aimed at quashing dissent, Amnesty International said in a report Wednesday.
"Enforced disappearance has become a key instrument of state policy in Egypt. Anyone who dares to speak out is at risk," Amnesty's Middle East and North Africa director, Philip Luther, said.
The London-based human rights group said abuses had surged since the military overthrew Islamist president Mohamed Morsi in 2013 and unleashed a crackdown on Islamist and secular dissidents.
Children were among those being kept at undisclosed locations for up to several months at a time "to intimidate opponents and wipe out peaceful dissent," the report said.
The report documents 17 cases, including five children, who had disappeared for periods of "between several days to seven months."
One of them, Mazen Mohamed Abdallah, who was 14 in September, had been subjected to "horrendous abuse" including "being repeatedly raped with a wooden stick in order to extract a false 'confession'," Amnesty said.
Another child of the same age when arrested in January, Aser Mohamed, "was beaten, given electric shocks all over his body and suspended from his limbs in order to extract a false 'confession'," the human rights watchdog said.
Egyptian authorities have denied they practise torture, but say there have been isolated incidents of abuse and those responsible have been prosecuted.
The foreign ministry accused Amnesty International of bias in its report, which relies on "sources expressing one side, and people and quarters who are hostile to the Egyptian state, while ignoring the judiciary's treatment of the cases mentioned."
The National Council for Human Rights, the country's official human rights watchdog, said on July 3 it had raised 266 cases of enforced disappearances with the interior ministry between April 2015 and the end of March.
Many of them have since been accounted for.
Amnesty's report "exposes... the collusion between national security forces and judicial authorities," Luther said.
He accused them of being "prepared to lie to cover their tracks" or failing to investigate torture allegations, "making them complicit in serious human rights violations".
An average of three to four people disappeared each day, Amnesty said, citing Egyptian non-governmental organisations.
It said they were usually detained at their homes by heavily armed security personnel, adding that investigations were rarely opened when family members complained.
Even then, authorities closed the investigations once they acknowledged the people were in the custody of the National Security Agency, the secret police apparatus.
Prosecutors "cruelly betrayed their duty under Egyptian law to protect people from enforced disappearances, arbitrary arrest, torture and other ill-treatment," Luther said.
Counter-terrorism was "being used as an excuse to abduct, interrogate and torture people who challenge the authorities."
An affiliate of the Islamic State group has waged an insurgency in Egypt's northern Sinai region since Morsi's overthrow that has killed hundreds of policemen and soldiers.