Alaa Abd El Fattah's letter was removed in secret from the Bab El-Khalq prison in Cairo by his wife, Manal Hassan. A full translation is published in the Guardian.
El Fattah describes being held in a cramped cell with eight other men, all of them detained on spurious charges. Some of them claim to have been tortured at the hands of the police.
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El Fattah compares the experience to his previous arrests under Hosni Mubarak's dictatorship:
"I never expected to repeat the experience of five years ago: after a revolution that deposed the tyrant, I go back to his jails?"
The only difference, he says, is that the state security prosecutor has been replaced by a military one.
As many as 12,000 civilians are believed to have faced military tribunals since Mubarak's fall in February, the Guardian says.
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The Supreme Council of the Armed Forces has been accused of multiple human rights abuses since taking over from the toppled regime. El Fattah was jailed on Sunday after denouncing the army's response to a recent Coptic Christian protest in Maspero, in which at least 27 people died.
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His arrest prompted mass protests in Cairo, reports Al Ahram, with thousands chainting "Freedom!" outside the walls of his prison:
Some fear Egypt's military rulers will seek to retain power permanently, writes the New York Times, pointing to recent posters of Field Marshal Mohamed Hussein Tantawi bearing the slogan "Egypt Above All".
Egyptians are due to vote in parliamentary elections later this month, to appoint the national assembly that will draw up the country's new constitution.
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