Despite the high number of detained protesters, the arrests are continuing across the country, a spokesman for the UN Human Rights Office said on Friday during a press conference in Geneva, Efe news reported.
"I'm extremely concerned about their physical treatment, violations of their right to due process, and the possibility that a significant number of them may be charged with offences that carry the death penalty, in addition to the conditions under which they are held," UN High Commissioner for Human Rights, Michelle Bachelet, said in a statement.
Spokesman Rupert Colville said the UN had received information that suggests "at least 208 people have been killed, including 13 women and 12 children."
However, Bachelet's office couldn't corroborate that data.
"In such circumstances, with so many reported deaths, it is essential the authorities act with far greater transparency," the UN High Commissioner for Human Rights said.
"They must undertake prompt, independent and impartial investigations into all violations that have taken place, including the killing of protesters and reported deaths and ill-treatment in custody; and those responsible must be held accountable.
"There appear to be multiple violations of the International Covenant on Civil and Political Rights, which Iran has ratified and is obliged to uphold."
Iran security forces, Revolutionary Guard and Basij militia, have responded to the protests not only with water cannons and tear gas, but in some cases with ammunition.
According to the UN Human Rights Office, several videos prove "severe violence was used against protesters, including armed members of security forces shooting from the roof of a justice department building in one city, and from helicopters in another."
"We have also received footage which appears to show security forces shooting unarmed demonstrators from behind while they were running away, and shooting others directly in the face and vital organs - in other words shooting to kill," Bachelet added.
"These are clear violations of international norms and standards on the use of force, and serious violations of human rights."
With thousands arrested by Iranian police, conditions in detention centres - prisons but also military barracks, sports facilities and schools - are very harsh.
The UN also denounced that Iranian journalists are being threatened, including those who work for media companies located outside the country.
Bachelet called on the Iranian authorities to release the protesters who have been arbitrarily detained to guarantee their right to defence and respect people's right to protest.
( With inputs from IANS )