An international media watchdog on Tuesday urged the United Arab Emirates to release a group of Internet activists held on charges of "opposing the constitution."
The Gulf state had arrested 18 "campaigners and rights activists" between July 16 and 19, said Reporters Without Borders (RSF). Only one of them has been released.
The group faces charges of "opposing the constitution and the basic principles of the UAE ruling system, in addition to having links and affiliations to organisations with foreign agendas," said RSF.
"The authorities must put an end to successive arrests of campaigners and human rights campaigners, which flagrantly violates basic freedoms," said the Paris-based watchdog.
"The authorities wilfully regard any sign of criticism of the system as a danger to national security in order to stifle dissent," it said. "These attempts at intimidation are doomed to fail."
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Thirty-one activists and campaigners have been in detention since March, according to the watchdog.
On Wednesday, Human Rights Watch made a similar appeal to Emirati authorities saying the country has "intensified a crackdown on peaceful political activists."
The arrests have mainly targeted members of UAE's Muslim Brotherhood-linked Reform and Social Guidance Association (Al-Islah).
The UAE had earlier this month announced it has dismantled a group plotting against state security without identifying their affiliation or the number of arrests.
Among those arrested last week is prominent lawyer Mohammed al-Roken, who has defended activists.
The UAE, a federation of seven emirates led by oil-rich Abu Dhabi, has not seen any pro-reform protests like those which have swept other Arab countries, including Gulf neighbours Bahrain and Oman, since last year.
But the government has increased its clampdown on voices of dissent and calls for democratic reforms.