The sister of Iran's supreme leader has slammed his "despotic" rule and thrown her support behind protests ignited by Mahsa Amini's death, in a letter published Wednesday by her son.

Protests have gripped Iran since Mahsa Amini, a 22-year-old Iranian of Kurdish origin, died on September 16 after her arrest in Tehran for an alleged breach of the Islamic republic's strict dress code for women.

Tehran says more than 200 people have been killed in the unrest, but Oslo-based non-governmental organisation Iran Human Rights says the country's security forces have killed at least 458 protesters, including 63 children.

Supreme leader Ayatollah Ali Khamenei, 83, has accused Iran's arch foe the United States and its allies of stoking the protests, which Tehran has sought to portray as "riots".

Rights activists say the demonstrations were sparked by anger over the repression of women, before expanding to include other grievances.

"I oppose my brother's actions," Khamenei's sister Badri Hosseini Khamenei, who is believed to be in Iran, said in a letter published online by her France-based son Mahmoud Moradkhani.

"I express my sympathy with all mothers mourning the crimes of the Islamic republic regime," from the time of its founder Ayatollah Ruhollah Khomeini "to the current era of the despotic caliphate of Ali Khamenei", she wrote.

"My concern has always been and will always be the people, especially the women of Iran," she added.

She accused the regime of bringing "nothing but suffering and oppression to Iran and Iranians" since it was established following the 1979 Islamic revolution that toppled the shah.

"The people of Iran deserve freedom and prosperity, and their uprising is legitimate and necessary to achieve their rights.

"I hope to see the victory of the people and the overthrow of this tyranny ruling Iran soon," she said.

Badri Hosseini Khamenei called on the powerful Islamic Revolutionary Guard Corps to "lay down their weapons as soon as possible and join the people before it is too late".

She lamented that "due to physical ailments" she was unable to take part in the protests.

"My brother does not listen to the voice of the people of Iran and wrongly considers the voice of his mercenaries and money-grabbers to be the voice of the Iranian people.

"He rightly deserves the disrespectful and impudent words he uses to describe the oppressed but brave people of Iran," she wrote.