Opponents of Iranian President Hassan Rouhani hold a protest outside the Iranian embassy in west London, Britain, January 2, 2018. REUTERS/Simon Dawson

GENEVA (Reuters) - The U.N. human rights chief called on Iran on Wednesday to rein in security forces to avoid further violence and respect the right of protesters to freedoms of expression and peaceful assembly.

Zeid Ra’ad al-Hussein, U.N. High Commissioner for Human Rights, said in a statement that more than 20 had been killed and hundreds arrested across Iran in the past week and he urged “thorough, independent and impartial investigations of all acts of violence that have taken place”.

Protesters “have a right to be heard”, he said. There must be “a concerted effort by the authorities to ensure that all security forces respond in a manner that is proportionate and strictly necessary, and fully in line with international law”.