Egyptian security forces routinely torture detainees as young as 15 years of age, according to testimony given to the BBC.
Former detainees – often held for being near a protest – have described being electrocuted, beaten and sexually abused by security personnel.
The army-backed interim government has detained some 20,000 people since it came to power last July.
A government official “categorically denied” torture had taken place.
“There might be some mistakes or transgressions in police stations however they don’t reach the level of torture,” said Gen Abu Bakr Abdel Karim from the interior ministry .
The accounts cannot be independently verified, but rights groups say torture and brutality in detention is common, says the BBC’s Orla Guerin in Cairo.
Torture and brutality perpetrated by the internal security agency under former President Hosni Mubarak fuelled opposition to his regime, which was overthrown in 2011.
Officials say this is a new Egypt, a country on a path to democracy, but human rights campaigners insist that systematic torture is back, our correspondent adds.
A 15-year-old boy told the BBC how he was accused of belonging to the Muslim Brotherhood, which has been the target of the new government’s crackdown.
He says he was electrocuted repeatedly.
“When they electrocuted me I used to fall down on the ground, and I could not stand up. At the same time they were beating me,” he said.
“And sometimes they would throw water to increase the voltage.”
Democracy activist Yassin Mohammed was arrested at a protest in January and held for 42 days.
The 19-year-old says he was tortured and electrocuted, and is speaking out on behalf of others who have had the same treatment.
“They took off my trousers, and then they put the wires on me. I was screaming and shouting. You feel that’s it. You are going to die,” he said.
The military-backed government took over after the Islamist President Mohammed Morsi was deposed after weeks of widespread protests.
Unrest has continued amid an ongoing crackdown on Mr Morsi’s followers.
On Friday at least three people were killed in clashes between security forces and supporters of Mr Morsi’s Muslim Brotherhood movement.
The bloodiest of the demonstrations was in the east of the capital, Cairo.
Two demonstrators and a female journalist covering the protests died when police moved in, using tear gas and live ammunition.