The UN's human rights agency has condemned the arrest of three members of a top rights group in Egypt.
Targeting activists was having a "profound chilling effect on an already weakened... civil society", it said.
The head of the Egyptian Initiative for Personal Rights, Gasser Abdel-Razek, was detained on Thursday, days after two colleagues. They are being held for suspected links with a terror group.
But Egypt has repeatedly denied accusations of human rights violations.
Mr Sisi led the military's overthrow of his democratically elected predecessor Mohammed Morsi in 2013 following protests against his rule.
The Egyptian Initiative for Personal Rights (EIPR) is an independent group whose work covers a variety of political, civil, economic and social issues.
It said in a statement on Friday that on the previous day security forces had arrested Mr Abdel Razek from his home in Maadi, south of Cairo, and taken him to an unknown location.
It said that its director of criminal justice, Karim Ennarah, was arrested on Wednesday in Dahab and its office manager, Mohammed Basheer, was detained in Cairo on Sunday.
Both men had been detained pending investigations, it said, adding that they faced charges including joining a terrorist group.
The EIPR described the arrests as a "clear and co-ordinated response" to its work, which includes monitoring places of detention and the issuance of death sentences.
It said Mr Abdel-Razek's arrest was "an attempt to put an end to organised and legitimate human rights work in Egypt".
Meanwhile Ravina Shamdasani, spokeswoman for the Office of the UN High Commissioner for Human Rights, said the arrests were feared to be part of a broader pattern of intimidation and silencing dissent.
"The use of sweeping counter-terrorism laws and vague charges such as 'joining a terrorist organization' and 'spreading false information' to harass and criminalise the work of human rights defenders is inconsistent with the rule of law and Egypt's obligations under international human rights law," she said.
"We are very concerned that the targeting of human rights defenders and other activists, as well as further restrictions on freedom of expression, association and peaceful assembly imposed in the country, are having a profound chilling effect on an already weakened Egyptian civil society."
France expressed "deep concern" over the arrest of Mr Basheer on Tuesday, and stressed that it maintained "a frank, exacting dialogue with Egypt on human rights issues".
The Egyptian foreign ministry on Wednesday rejected France's "interference in an Egyptian internal affair". It also stressed that Egypt respected the rule of law and equality before it.