The gesture is in solidarity with protests in Iran over the death of a young woman in custody.
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Nazanin Zaghari-Ratcliffe, a British-Iranian national who spent six years in jail in Iran, has cut her hair in solidarity with Iranian protesters.
Iran has seen nearly two weeks of demonstrations over the death of a young woman in police custody.
Mahsa Amini, 22, died after allegedly breaching rules requiring women to cover their hair with a hijab.
In line with protesters in Iran, Ms Zaghari-Ratcliffe is seen cutting her hair in footage give to BBC Persian.
At the end of the video, she says: “For my mother, for my daughter, for the fear of solitary confinement, for the women of my country, for freedom.”
Ms Zaghari-Ratcliffe was arrested in Iran in 2016 on spying charges which she denied.
After a sustained campaign, and negotiation between the British and Iranian governments, she was released in March and returned to the UK.
Ms Amini was arrested by Iran’s morality police in Tehran on 13 September. She collapsed after being taken to a detention centre, and later died in hospital.
Police said her death was caused by sudden heart failure, but Ms Amini’s family have dismissed this and alleged that she was beaten by officers.
Her death sparked anti-government protests, which have now spread to more than 80 cities and towns across Iran.
Iran Human Rights, an organisation based in Norway, says at least 76 protestors have been killed by Iranian security forces.
State media put the figure at 41, including several security personnel, and blamed “rioters”. Hundreds of people have been arrested.
Women have been filmed burning their hijabs and cutting their hair during the demonstrations, which activists have said is an ancient Persian symbol of protest.