Hollywood actors, renowned writers urge Egypt to free activist
"We call on the Egyptian authorities to immediately release Sanaa...and all those detained for peacefully exercising their rights," the letter said.
Seif, 26, was arrested outside the public prosecutor's office and driven away in an unmarked minivan.
She was charged with "spreading false news", "inciting terrorist crimes" and "misuse of social media", her lawyers said.
The letter was signed by more than 200 people, including actors Danny Glover, Maggie Gyllenhaal and Thandie Newton and leading writers Noam Chomsky, Arundhati Roy and Nobel Literature laureate J. M. Coetzee.
"Sanaa is the latest in a seemingly unending series of arrests that have come to define President Abdel Fattah al-Sisi's rule," the signatories said.
Seif, a film editor who worked on the Oscar-nominated documentary The Square, is the sister of jailed activist Alaa Abdel Fattah, a prominent figure in the Arab Spring uprising that unseated longtime autocrat Hosni Mubarak in 2011.
The family had been at the prison in hope of receiving a letter from Alaa, who was imprisoned last September after rare, small-scale protests demanding Sisi's ouster prompted thousands of arrests.
"Tens of thousands of politically motivated detentions have been documented by human rights organisations... Meanwhile, courts and prosecutors have been extending the pre-trial detention of detainees without their presence in court," the open letter said.
Under Egyptian law, defendants can be remanded in custody for up to two years but authorities routinely prolong detentions indefinitely.
The open letter described how detainees are "trapped in endless cycles of administrative detention, renewed indefinitely, without ever going to trial".
"We call on the Egyptian government to end the abuse of pre-trial detention," it said.
Advocacy groups including Amnesty International and Human Rights Watch endorsed the letter.
Egypt has targeted dissidents with a persistent crackdown since then armed forces chief Sisi toppled Islamist president Mohamed Morsi in 2013.
Thousands have been swept up in the campaign, not only Islamists but also secular activists, lawyers, journalists, film-makers and most recently social media influencers.
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