Amnesty International urged UAE authorities on Tuesday to release activists or provide them with fair trials, saying 50 people have been arrested in a clampdown on peaceful dissent.
"UAE authorities must halt the ongoing crackdown on peaceful dissent," the London-based organisation said, adding that 35 men are believed to have been detained since July 16, bringing the total of arrests since March to 50.
"The whereabouts of all 35 are unknown, and they are thought to be at risk of torture or other ill-treatment, which Amnesty International has previously documented in the UAE," it said.
On July 15, the UAE announced that it had dismantled a group it said was plotting against state security and challenging the constitution of the Gulf state without identifying their affiliation or the number of arrests.
"This wave of arrests is deeply disturbing and appears to be part of an ongoing attack on the rights to freedom of expression, association and assembly in the UAE," said Ann Harrison, Amnesty's deputy Middle East and North Africa director.
She urged authorities to disclose the location of the 35 men and allow them immediate contact with their families.
"Incommunicado detention is known to facilitate torture and other ill-treatment," she said.
Lawyers Mohammed al-Roken, Mohammed al-Mansoori and Salem al-Shehhi "are believed to be prisoners of conscience, held solely in connection with the peaceful expression of their views or for carrying out their professional duties," she said.
They "must be released immediately and unconditionally."
"Any others held must be released unless they are to be tried promptly on an internationally recognisable criminal charge in full accordance with international fair trial standards," she said.
Last week, Dubai police chief General Dahi Khalfan accused the Muslim Brotherhood, which is the emerging force in the Arab world after the Arab Spring uprisings, of plotting against Gulf monarchies, claiming the detainees were linked to the group.
The banned Islamist group Al-Islah, to which many of the detainees are said to belong, called on Monday for the release of the activists and for an end to "all security harassment against citizens."
It condemned "false accusations" of challenging the political system, renewing allegiance to the leadership of the federation of seven hereditary sheikhdoms.
The UAE has not seen the kind of pro-reform protests that have swept other Arab countries, including Gulf neighbours Bahrain and Oman, since last year.