Pope Francis on Monday condemned Iran for using the death penalty against demonstrators demanding greater respect for women.

The pope's remarks, made in his yearly speech to diplomats accredited to the Vatican, were his strongest since the start of nationwide protests in Iran following the death last September of 22-year-old Kurdish Iranian woman Mahsa Amini in police custody.

"The right to life is also threatened in those places where the death penalty continues to be imposed, as is the case in these days in Iran, following the recent demonstrations demanding greater respect for the dignity of women," Francis said.

"The death penalty cannot be employed for a purported state justice, since it does not constitute a deterrent nor render justice to victims, but only fuels the thirst for vengeance," he said.

He then repeated an appeal for an end to the death penalty worldwide, saying it is " always inadmissible since it attacks the inviolability and the dignity of the person".

Four protesters have been executed in the aftermath of the unrest in Iran.