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Follow UsAhmed Mansoor had received death threats online, which he said were for joining a petition demanding wider political representation and legislative powers for the Federal National Council, a parliamentary-style body.
Two others detained earlier this week
Two other men, a blogger and a political commentator, were detained earlier this week, the lawyer for all three said, though police have denied arresting Fahad al-Shehhi or Nasser bin Ghaith.
The lawyer, Abdulhamid al-Kumaity, said police had told him Mansoor was charged with possession of alcohol, but he said there was no evidence he was carrying any.
Alcohol is available in hotels and bars in Dubai but the authorities have the power to arrest Muslims for consumption or possession on the basis of Islamic sharia law.
A day before his arrest, Mansoor had sent an email to colleagues and friends saying he was worried police would try to plant something in his car to create a reason for his arrest.
Dubai's police spokesman was unavailable for comment, but local newspaper The National cited the Dubai police chief as saying Mansoor was arrested in connection with a criminal case.
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Mansoor and Shehhi are members of the online discussion forum UAE Hewar, where participants have often debated and criticised local politics. Bin Ghaith wrote articles online that were critical of Gulf Arab governments.
Online campaigns
The UAE, a federation of seven emirates headed by ruling families, does not allow direct elections or political parties. It is a U.S. ally which shares Washington's concerns about rising Iranian influence in the region.
Online activists have attacked Mansoor. On Facebook, a page called "Ahmed Mansoor, a traitor without a nation," posts comments and video clips denouncing him.
"The time has come for people like him to go down in history defeated with their heads bowed," one post read.
Rival activists and students have posted a picture of Mansoor, in a white robe and headdress, with the words "Freedom for Ahmed Mansoor" emblazoned in red on the bottom.
As the world's third largest oil exporter, with ample resources and generous spending programmes, the UAE has been spared the mass protests that have swept the Arab region and ousted the leaders of Tunisia and Egypt. Per capita income is over $47,000 per year, among the world's highest.
State media said last month the UAE would invest $1.6 billion on infrastructure in its less-developed northern emirates, whose citizens have benefited least from the vast oil wealth in the capital Abu Dhabi and from Dubai's trade- and property-fuelled development.