Four people have been sentenced to death on the charge of "enmity against God" in connection with the recent anti-government protests in Iran.

Revolutionary Courts in Tehran said one of the unnamed "rioters" hit and killed a policeman with his car, the judiciary's Mizan news agency said.

The second possessed a knife and a gun, and the third blocked traffic and caused "terror", it alleged.

The fourth was convicted of a knife attack, Mizan reported late on Tuesday.

"Protesters don't have access to lawyers in the interrogation phase, they are subjected to physical and mental torture to give false confessions, and sentenced based on the confessions," the director of Norway-based Iran Human Rights, Mahmood Amiry-Moghaddam, told AFP news agency.

Although the judiciary did not disclose the identities of the five individuals sentenced to death, Amnesty International said information about their charges had led human rights activists to believe they were Mohammad Ghobadlou, Manouchehr Mehman Navaz, Mahan Sedarat Madani, Mohammad Boroughani and Sahand Nourmohammad-Zadeh.

It added that they were among at least 21 detainees charged with security-related offences that are punishable by death under Iran's Sharia-based legal system, including "enmity against God" and "corruption on Earth".

At least 348 protesters have been killed and 15,900 others arrested in a crackdown by security forces on what Iran's leaders have portrayed as foreign-backed "riots", according to the Human Rights Activists News Agency (HRANA), which is also based outside the country.

The women-led protests against clerical rule erupted after the death in custody three months ago of Mahsa Amini, a 22-year-old woman who was detained by morality police for allegedly breaking the strict rules on hijabs.