Mir Hossein Mousavi says he does not believe country's current system still works, while the country's Supreme leader pardons "tens of thousands" of prisoners, according to state media.
A long-detained opposition leader in Iran has called for a nationwide referendum about whether to write a new constitution for the country amid nationwide protests.
Mir Hossein Mousavi's call, posted late on Saturday by the opposition Kaleme website, included him saying he did not believe Iran's current system.
Iranian security forces have been cracking down on demonstrators since protests began in September, after the death of Mahsa Amini under police custody.
Mousavi is also calling for the formation of a constitutional assembly of “real representatives” to write a new constitution.
He and his wife have been under house arrest for years after his disputed presidential election loss in 2009 led to the widespread Green Movement protests that security forces also put down.
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In another development, Iran's Supreme leader pardoned "tens of thousands" of prisoners, including many arrested in recent anti-government protests over security-related charges, state media reported on Sunday.
"Prisoners not facing charges of spying for foreign agencies, having direct contact with foreign agents, committing intentional murder and injury, committing destruction and arson of state property, or not having a private plaintiff in their case will be pardoned," state media said.
The pardons were announced in honour of the anniversary of the 1979 Iranian revolution.
According to the HRANA activist news agency, about 20,000 people have been arrested over the anti-government protests that started in September last year.
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