Human rights violations in Iran are as poor as at any time in the past 20 years, the human rights group Amnesty International has said.
Its report, which followed June's disputed presidential election, included allegations of torture, rape and unlawful killings.
Amnesty called on Iran's Supreme Leader Ayatollah Ali Khamenei to allow United Nations human rights experts to visit the country to help carry out an investigation.
Official inquiries to date "seemed to have been more concerned with covering up abuses than getting at the truth", it added.
Iran has dismissed previous criticism of its human rights record.
"Members of militias and officials who have committed violations must also be promptly held to account and on no account should anyone be executed," Deputy Director of Amnesty International's Middle East and North Africa Programme Hassiba Hadj Sahraoui said in a statement.
The report claimed patterns of abuse before, during and, particularly, after the election, when authorities deployed the the Basij militia and Revolutionary Guards to suppress mass protests against its disputed outcome.
Mass demonstrations against the re-election of hardline President Mahmoud Ahmadinejad plunged the Islamic Republic into crisis, but opposition rallies no longer muster the huge crowds that flooded the streets immediately after the June 12 ballot.
Defeated candidate Mirhossein Mousavi has alleged the vote was rigged. Some of those detained during the protests have since been forced to flee the country, the report said.
One former detainee says he was held at the Kahrizak detention centre for about 58 days, kept in a shipping container throughout and only allowed to contact his family after 43 days, the report added.
"Anyone who is arrested or detained must be protected from torture or other ill-treatment, prisoners of conscience must be released and those convicted after unfair trials - including the 'show trials' which made a mockery of justice - must have their cases reviewed, or be released," Sahraoui said.
"All death sentences should be commuted, and others not yet tried must receive fair trials."
Reuters