In a statement, the UN human rights office also said it had obtained "verified video footage" showing security forces firing on protesters, apparently with intent to kill.
The rights office added that it had "information suggesting that at least 208 people were killed" during the unrest, supporting a toll previously given by Amnesty International.
"There are also reports, which the UN Human Rights Office has so far been unable to verify, suggesting more than twice that number killed," the statement added.
The UN High Commissioner for Human Rights, Michelle Bachelet, said video obtained by her office shows "severe violence was used against protesters."
"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 said.
Additional video material shows "armed members of security forces shooting from the roof of a justice department building" in the city of Javanrud, west of Tehran in Kermanshah Province, as well as gunfire from helicopters in Sadra, in Fars Province.
The protests began on 15 November following a surprise hike in fuel prices.
Iran has yet to give overall figures for the number of people killed or arrested when security forces moved in to quell the unrest that saw buildings torched and shops looted.
Bachelet charged that "many of the arrested protesters have not had access to a lawyer," while raising alarm over "reports of severe overcrowding and harsh conditions in detention centres, which in some cities include military barracks, sports venues and schools."
"I urge the authorities to immediately release from detention all protesters who have been arbitrarily deprived of their liberty," she further said.
Iran has blamed the violence that broke out during the protests on "thugs" backed by its foes the United States, Israel and Saudi Arabia.
Tehran has also dismissed the high death tolls reported by foreign sources as "utter lies".
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