Iranian security forces committed a "catalogue of shocking human rights violations" against those detained in a crackdown on nationwide protests last November, Amnesty International says.
Dozens of men and women told the human rights group they were beaten, flogged, given electric shocks, or sexually abused to extract bogus confessions.
More than 7,000 people - including children as young as 10 - were arrested in the crackdown. Hundreds more died.
Iran has not commented on the report.
But it has previously rejected criticism of its human rights record as baseless.
November's protests erupted after the Iranian government increased the price of petrol by 50%.
The decision was met with widespread anger in a country where the economy was already reeling as a result of US sanctions, and hundreds of thousands of people took to the streets.
Iran's supreme leader denounced the protesters as "evil-doers" incited by "foreign enemies", and security forces launched a deadly crackdown.
Amnesty has documented the cases of 304 men, women and children it says were killed by security forces over five days - most from gunshot wounds. Iran's interior minister has indicated that the death toll was below 225.
At least 7,000 other people were arrested, according to the spokesman for the Iranian parliament's national security committee, although media reports suggest the figure was far higher.
Amnesty's new report, entitled "Iran: Trampling Humanity", gathered the testimonies of 60 detainees and 14 individuals who had either witnessed or investigated reported violations.
The detainees alleged that torture was routinely used to elicit "confessions" and incriminating statements, not just about their involvement in the protests, but also about their alleged associations with opposition groups, human rights defenders, media outside Iran, as well as with foreign governments.
Amnesty said the torture methods included "waterboarding, beating, flogging, electric shocks, pepper-spraying genitals, sexual violence, mock executions, pulling out nails and solitary confinement, sometimes for weeks or even months".
One man who was subjected to electric shocks recounted: "It felt like my entire body was being pierced with millions of needles. If I refused to answer their questions, they would raise the voltage levels and give me stronger electric shocks... The torture has had lasting effects on my mental and physical health."
Another man was suspended from his hands and feet from a pole in a painful method his interrogators referred to as "chicken kebab".
"The pain was excruciating. There was so much pressure and pain in my body that I would urinate on myself," he said. "My family know that I was tortured, but they don't know how I was tortured."
Amnesty said hundreds of detainees were convicted of "vague or spurious national security charges" following "grossly unfair trials which were presided over by biased judges behind closed doors" and which relied on torture-tainted "confessions".
Many were sentenced to prison or flogging, and several were sentenced to death.