CAIRO, April 20 (Reuters) - A U.S.-based human rights group pressed Egypt on Friday to release a prominent Muslim Brotherhood activist, blogger and journalist, calling his detention a blow to free speech.

Abdel Moneim Mahmoud was detained at Cairo airport a week ago as he tried to travel to Sudan to report for London-based Arabic satellite channel al-Hiwar on human rights in the Arab world.

Mahmoud runs the Ana Ikhwan (I am a Brother) blog in Egypt, the Arab world's most populous nation, and has criticised the government over human rights abuses.

"Once again, the Egyptian government is prosecuting a journalist because he has reported on human rights abuses in the country," Sarah Leah Whitson, Middle East and North Africa director at Human Rights Watch, said in a statement.

"The government should focus its energies on ending the abuses, not silencing those who expose them," she added.

Mahmoud's arrest came amid a government clampdown on the opposition Brotherhood. Rights groups have also complained that Egypt was becoming less tolerant of public shows of dissent, especially on the Internet.

Mahmoud, 27, also helps run the Brotherhood's English website (www.ikhwanweb.info), and has been at the forefront of a campaign to release dozens of Muslim Brotherhood members currently in detention in Egypt, where the group is formally banned and members subject to sporadic arrest campaigns. The Brotherhood is Egypt's most powerful opposition group, holding about a fifth of the seats in the country's parliament through members elected as independents.

Analysts say the government wants to stop the Brotherhood from making further political gains that could jeopardise the traditional dominance of President Hosni Mubarak's ruling National Democratic Party.

Hundreds of Brotherhood activists have been arrested in recent months, including third-in-command Khairat el-Shatir, who was transferred to a military trial along with 39 other members on charges including money-laundering and terrorism. The group has strongly denied the accusations.

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