Egyptian pro-democracy activist Alaa Abdel Fattah 'beaten' in jail
Abdel Fattah rose to prominence during the 2011 Arab Spring, when a series of uprisings swept the Middle East, leading to long-time Egypt President Hosni Mubarak being toppled.
He was among more than 2,900 people who activists say were arrested since the small and rare 20 September protests that demanded President Abdel-Fattah al-Sisi step down.
Abdel Fattah told his lawyers that since his arrest, he was subjected to several violations including being blindfolded, insulted, slapped, kicked and threatened never to be released from Cairo's most notorious prisons, where he is being held, a statement from his family read.
Mona Seif, Abdel Fattah's sister, told The Associated Press that he had filed a legal complaint about all the alleged abuses with the State Security prosecutors during a hearing on Wednesday to renew his pre-trial detention.
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His family fears the retaliation of prison authorities.
"By returning to the same prison with the same people who threatened him, we are worried that he might face more violations and more torture," said Seif, a staunch human rights advocate.
She said the family would remain together outside prison gates until they are allowed to go in to see him and are "assured that he is fine".
There was no immediate comment from Egyptian authorities about the allegations.
Abdel Fattah was released in March, after five years in prison for taking part in a peaceful protest against military trials of civilians.
To many, his imprisonment after al-Sisi rose to power - at a time when authorities imposed draconian laws banning public gatherings and unauthorized demonstrations - was another sign of Egypt's return to autocratic rule.
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He was arrested last month from a Cairo police station, where he was required to check in every night under the legal terms of his release.
This time, Abdel Fattah is facing several charges, including belonging to a terrorist organization and using the social media to spread false news that could threaten national security, said his sister.
Nine Egyptian human rights groups released a statement denouncing the alleged ill-treatment of Abdel Fattah and calling on the UN to examine the situation of human rights in Egypt.
"The continuous attempts to terrorize, crackdown on and punish everyone indiscriminately can only lead to more instability in the country," read the statement.
Shortly after Abdel Fattah's family posted their statement Thursday, the head of the EU parliament's subcommittee on human rights, Marie Arena, urged Egyptian authorities to release him, as well as his lawyer, who was also arrested along with the activist.
Arena tweeted that she is "dismayed that one of the most prominent Egyptian human rights defenders" is in danger after being "subjected to death threats and torture by state security officers."
Agencies contributed to this report