Torture and summary executions are rife in secret prisons in Syria run by the Islamic State in Iraq and the Levant (ISIS), Amnesty International says.
A new report says al-Qaeda affiliate had committed serious rights abuses, including some amounting to war crimes.
Isis is one of the main jihadist groups fighting government forces, and has a strong presence in the north of Syria.
The rights group says "the people of al-Raqqa and Aleppo are suffering under a new form of tyranny imposed by Isis".
"Those abducted and detained by ISIS include children as young as eight who are held together with adults in the same cruel and inhuman conditions," said Amnesty's regional director Philip Luther.
'Reign of terror'
The report alleges that in areas they control, ISIS forces had committed numerous serious rights abuses, including some that amount to war crimes, such as abductions, arbitrary detention, torture and other ill-treatment, and unlawful killings.
Ten former detainees interviewed by an Amnesty researcher in the past month recounted "a shocking catalogue of abuses", the report said, including being flogged with rubber generator belts or cables, tortured with electric shocks or forced to adopt a painful stress position in which their wrists were secured together over one shoulder.
Some of those held by ISIS were suspected of theft; others of "crimes" against Islam, such as smoking, alcohol consumption or sex outside marriage, the report added. Others were seized for challenging ISIS's rule or because they belonged to rival rebel groups.
Amnesty said it was told that several children had received severe floggings. Two detainees described how they had seen a 14-year-old get more than 90 lashes during interrogation at Sadd al-Baath, an ISIS prison beside a dam on the Euphrates river at al-Mansoura.
Amnesty said the local Sharia court judge at Sadd al-Baath invariably appeared wearing an explosives belt and had "instituted a reign of terror over its detainees".
Former detainees accused him of presiding over grotesquely unfair trials lasting no more than a few minutes, and of handing down death penalties which were subsequently carried out.
"After years in which they were prey to the brutality of the Assad regime, the people of Raqqa and Aleppo are now suffering under a new form of tyranny imposed on them by ISIS, in which arbitrary detention, torture and executions have become the order of the day," Mr Luther said.
Raqqa, a city sheltering around a million people, is under the full control of ISIS.
Smaller but better funded than other rebel groups, it has historically been made up of foreign jihadists from Arab countries - particularly Iraq, but also Libya, Tunisia, and Saudi Arabia - Russia's north Caucasus, and Europe.
But activists from Raqqa told the BBC's Newsnight that it was now attracting more and more Syrian recruits.
In the past few months, ISIS has made significant advances, largely at the expense of rebel brigades affiliated to the Western-backed Free Syrian Army, and controls a wide swathe of northern Syria close to the Turkish border.
Amnesty called on the international community to help to block the flow of arms to Isis and other armed groups implicated in war crimes and serious rights abuses.
It also renewed its appeal to the Syrian government to "end its violations of human rights and international law, including the use of torture in its own detention centres".
The United Nations says more than 100,000 people have been killed since the uprising against President Bashar al-Assad began in March 2011. More than nine million others have been forced from their homes.