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STORY: Iran's clerical rulers have been grappling with the biggest nationwide unrest in years, with support spreading abroad in western capitals.

Here's what you need to know.

[How the protests grew]

People across Iran have joined protests since the death of Mahsa Amini in the custody of morality police who detained the 22 year old in Tehran on September 13 for "inappropriate attire."

The officers, who enforce strict hijab rules, rejected accusations she was beaten, saying she fell ill as she waited with other detained women.

Students and universities have become a focal point of the demonstrations; the greatest challenge to the country's clerical elite in years, amid calls for the downfall of the Islamic Republic.

But many Iranians, including celebrities, have taken part or offered support for the protests.

Three weeks in, rights groups say thousands have been arrested and hundreds injured, with over 150 deaths.

[Response of Iranian authorities]

The Iranian authorities are waging a deadly crackdown - even if observers do not believe the Shi'ite clerical establishment is close to being toppled.

President Ebrahim Raisi has ordered an investigation into Mahsa Amini's death.

Ayatollah Khamenei says it "deeply broke my heart", but he's given his full backing to security forces.

The Basij, a volunteer militia affiliated with Iran's Revolutionary Guards, has played a big role in quelling the unrest, alongside riot police.

Video said to have been filmed at a school in Shiraz, shows about 50 female pupils shouting "Basij get lost."

Authorities have reported numerous deaths among the security forces.

State-organized counter protests have been mounted.

Iran has also accused adversaries, including the U.S. and Britain, of orchestrating the unrest.

[International reaction]

Amini's death and the crackdown are compounding international tensions, already heightened by stalled negotiations on reviving Tehran's 2015 nuclear deal.

Ravina Shamdasani is a UN human rights spokesperson:

"There are reports that Miss Amini was beaten on the head with a baton, and her head was banged against the vehicle by so-called morality police. Authorities have stated that she died of natural causes // Miss Amini’s tragic death and allegations of torture and ill treatment must be promptly, impartially and effectively investigated by an independent competent authority, that ensures, in particular, that her family has access to justice and the truth."

Female protesters have cut their hair during rallies around the world.

In France, leading actresses have cut locks of their hair, and a Swedish member of the European parliament snipped off her ponytail.

The U.S. has said it will impose "further costs" on Iranian officials responsible for violence against protesters.