Human Rights Watch on Saturday urged authorities in the United Arab Emirates either to charge a group of Egyptians held over suspected links with Egyptian President Mohamed Morsi's Muslim Brotherhood or release them.
"If the UAE government can show the Egyptian detainees have engaged in criminal behavior, why hasn't it charged them with a crime," asked Nadim Houry, HRW's deputy Middle East director.
"Authorities should stop this shameful practice of arresting people only to hold them without charge for months on end," he said.
Citing relatives of seven of them, HRW added that the "authorities have not granted access to lawyers or in-person visits with family members."
Last month, local media reported that UAE authorities had arrested 11 Egyptians residing in the country on suspicions of links to the Brotherhood.
The HRW statement spoke of 13, and it was not immediately clear why there is a discrepancy in the numbers.
The group were arrested between November 2012 and January 2013, HRW said, and held incommunicado until February 7 when they were allowed to call relatives inside the Gulf country.
Among them are doctors, engineers and university professors who have been living and working in the UAE for many years.
They are suspected of leading an expatriate Brotherhood cell that collected sensitive information and had links to Emiratis in custody on suspicion of plotting against national security.
The "arbitrary" detention of the Egyptians follows "similar violations of the rights of 94 Emiratis also detained arbitrarily for months without charges amid a broader crackdown on freedom of expression and critics of the government," said HRW.
The 94 Islamists, accused of plotting against the Gulf state, will appear before the Federal Supreme Court on March 4, according to local media.
The UAE, a federation of seven emirates led by oil-rich Abu Dhabi, has not seen any of the widespread pro-reform protests that have swept other Arab countries, including fellow Gulf states Bahrain and Oman.
But authorities have stepped up a crackdown on voices of dissent and calls for democratic reform.
Dubai police chief General Dahi Khalfan has accused the Muslim Brotherhood of plotting against Gulf monarchies.
The case of the Egyptians has sparked a sharp deterioration of relations between Abu Dhabi and Cairo, already under strain since Morsi's election as Egyptian president last June.
The Gulf country, where membership of political parties is banned, has rejected a request from Egypt for the release of its nationals.