Director of criminal justice at advocacy group Egyptian Initiative for Personal Rights (EIPR), Karim Ennarah, was arrested by the Egyptian authorities on Wednesday, days after the leading local human rights group's office manager Mohamed Basheer was detained.
Both EIPR and Human Rights Watch believe the arrests are due to a meeting held with several western diplomats at the group's Cairo offices on 3 November to discuss human rights in Egypt.
EIPR director Gasser Abdel Razek said he was shocked that "a security force would feel threatened by a meeting with ambassadors", Egyptian outlet Mada Masr reported.
Human Rights Watch Egypt researcher Amr Magdi said "these arrests are a slap in the face from the Egyptian government to all these diplomats and European countries who attended this meeting", in comments shared by The Telegraph.
"They are saying, 'We don't care, you cannot protect anyone, and we can arrest anyone, including those who are well recognised human rights defenders,'" he said.
The ambassadors of the UK, Germany, Belgium, Denmark, Finland, France, Italy, the Netherlands, Spain, and Switzerland all attended the meeting, according to EIPR.
The arrests represent an "an alarming and dangerous escalation" of a sweeping crackdown on dissent under President Abdel Fattah al-Sisi, Magdi added.
"Kareem and Mohammed were arrested for their work, for their mere association with the EIPR, it's not about attending a protest or an article they wrote," he said.
"That's what's different here from previous arrests of human rights activists. We fear that more staffers will be arrested."
Karim Ennarah was arrested on Sunday while on vacation in Dahab, South Sinai. EIPR did not mention any possible accusations against Ennarah.
Mohamed Basheer, who was arrested by security sources on Sunday, is being held on charges including "joining a terror group" and "spreading false news", EIPR said.
EIPR said Basheer was questioned by the Supreme State Security Prosecution (SSSP) about the organisation's work and a visit earlier this month to its Cairo office "by a number of ambassadors and diplomats" to discuss human rights.
Basheer was placed in pre-trial detention for 15 days and will be questioned at a later date, EIPR said.
Pre-trial detention can last up to two years under Egyptian law, but the period is often extended.
Amnesty International also slammed Ennarah's arrest as a "chilling escalation of the Egyptian authorities' crackdown on civil society".
"These arrests, following a meeting at EIPR with Western diplomats, serve a heavy blow against the legitimate work of human rights defenders," Amnesty said on Twitter.
No comment was forthcoming from the authorities on the two arrests.
Rights groups estimate that some 60,000 detainees in Egypt are political prisoners.
These include secular activists, journalists, lawyers, academics and Islamists arrested in a sweeping crackdown on dissent under President Abdel Fattah Al-Sisi.
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