Ruhollah Zam was hanged on Saturday after Iran's supreme court upheld his death sentence passed in June over his role in protests during the winter of 2017-18.
The UN rights chief has voiced outrage at Iran's execution on the weekend of opposition figure Ruhollah Zam, and urged Tehran to halt its "alarming and increasing" use of the death penalty.
"I am appalled at the execution in Iran on 12 December of Ruhollah Zam," Michelle Bachelet said in a statement on Monday.
"His death sentence and execution by hanging are emblematic of a pattern of forced confessions extracted under torture and broadcast on state media being used as a basis to convict people."
Zam was hanged on Saturday after Iran's supreme court upheld his death sentence passed in June over his role in protests during the winter of 2017-18, among other charges.
The dissident, who ran a Telegram channel widely followed in the protests, had lived in Paris for several years after being given refugee status and residency in France.
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But activists say he was held after travelling to Iraq from Paris in October 2019 in circumstances that remain unclear, with some campaigners accusing Tehran of abducting him.
In her statement, Bachelet also highlighted "serious concerns that the apprehension of Zam outside the territory of Iran could amount to an abduction and that his subsequent transfer to Iran for trial may not have respected due process guarantees."
The UN High Commissioner for Human Rights stressed that Zam was among many activists and protesters in Iran who have been "sentenced to long prison terms or given the death penalty for exercising their human rights after trials that failed to comply with international standards regarding due process."
"These constitute serious violations of Iran's obligations under the International Covenant on Civil and Political Rights, including the right to freedom of opinion and expression, and the right to life," she said.
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'Increasing use of death penalty' criticised
Bachelet urged Iranian authorities to "immediately halt their alarming and increasing use of the death penalty and vague national security charges to suppress independent voices and dissent in Iran."
She also called on Tehran to "immediately free all those arbitrarily detained for exercising their human rights."
The UN rights chief said she was opposed to the death penalty in all cases, insisting it was "simply irreconcilable with human dignity, is too often arbitrary in its application, and errors can be never be made good again."
If countries insist on using the penalty, she stressed that under international law, it can only be applied for the most serious crimes, and only after a fair trial and access to appeal and the right to seek clemency.
These, she said, are "cumulative conditions clearly not met in Ruhollah Zam’s case."
US Secretary of State Mike Pompeo also condemned the execution of Zam, calling it "unjust, barbaric."