At least 304 people were killed in Iran during a three-day crackdown against protests across the country in November, according to updated numbers published by Amnesty International on Sunday night and quoted by AFP.
The rights group had earlier estimated 208 deaths, including two youths aged 15 and 17.
It is difficult to obtain official numbers in large part due to the fact that the Iranian regime blocked access to the internet during the crackdown.
Amnesty said it collected "harrowing testimony" suggesting that after authorities "massacred" protesters, they orchestrated a "wide-scale clampdown" to cover up the deaths.
"Iran's authorities are carrying out a vicious crackdown following the outbreak of nationwide protests on 15 November," the London-based rights watchdog said in a statement quoted by AFP.
"Thousands of protesters as well as journalists, human rights defenders and students" were arrested, Amnesty said, "to stop them from speaking out about Iran's ruthless repression".
The unrest erupted on November 15, hours after it was announced that the price of gas would rise to 15,000 rials per liter (12 US cents) from 10,000 for the first 60 liters, and to 30,000 rials for any extra fuel bought after that each month.
Authorities restored order within days, but so far have confirmed just five deaths, including four members of the security forces killed by "rioters".
A New York Times report published two weeks ago and based on witness accounts and videos, said security forces responded to the protests by opening fire on unarmed protesters, largely unemployed or low-income young men between the ages of 19 and 26.
"Independent sources" told Amnesty that a month after the unrest, "security forces are still carrying out raids across the country to arrest people in their homes and places of work."
Adolescents as young as 15 have been "detained alongside adults", Amnesty said.
With dozens held in "incommunicado detention" and others in "conditions amounting to enforced disappearance", some detention centers face "severe overcrowding", Amnesty claimed.
The organization called on Tehran to "urgently and unconditionally release all those who have been arbitrarily detained".
Iran blamed the unrest on "thugs" backed by its foreign enemies, including the US, Israel and the People's Mujahedeen of Iran, an exiled armed opposition group it considers a "terrorist" cult.
Iranian Supreme Leader Ayatollah Ali Khamenei claimed following the crackdown that his country had foiled a "very dangerous" plot in the violent demonstrations.
Iran’s Revolutionary Guards arrested about 100 leaders of the protests and said they would act to severely punish them.
Iranian Vice-President Eshaq Jahangiri warned regional countries of “dire consequences” if it is proven that they meddled to stoke unrest in Iran.