(Adds details and background)

By Ibrahima Sylla

NOUAKCHOTT, June 5 (Reuters) - A Mauritanian court on Tuesday acquitted 20 Islamists on charges of trying to set up an al Qaeda-linked group in the Saharan Islamic state.

One suspected militant was found guilty of possessing illegal arms and false documents and jailed for two years after the two-week trial in Nouakchott. Four of the group had been tried in absentia and the rest were freed on their acquittal.

Family members of those acquitted celebrated the verdict by sounding car horns while women made the ululating cries that are a traditional sign of rejoicing in this Arab-African nation on the Sahara's western edge.

The trial was the first of its kind since a civilian president took office last month, following elections in March that sealed a democratic handover by a military junta.

Defence lawyers, who had accused police of torturing the suspects to obtain statements, welcomed the verdicts as a victory for justice. Police had denied they tortured the suspects, some of whom were arrested in 2005.

"I am very happy about this decision by the court which has established the innocence of the accused, some of whom have been in prison for more than 28 months in difficult conditions and putting up with all kinds of humiliation," said lawyer Fatmata Mbaye, who also heads the Mauritanian Human Rights Association.

Some of the men, who included more than a dozen imams or religious teachers, had been accused of receiving training from al Qaeda's North Africa wing, previously known as the Salafist Group for Preaching and Combat (GSPC).

The Algerian-based GSPC changed its name to the al Qaeda Organisation in the Islamic Maghreb. It claimed responsibility for triple suicide bombings in Algeria last month and is on a U.S. list of terrorist organisations.

Some of the accused were arrested in April 2005 under the government of then-president Maaouya Ould Sid'Ahmed Taya, who was deposed in a military coup in August that year. The junta organised polls and handed over last month to an elected civilian president, Sidi Mohamed Ould Cheikh Abdallahi.

Taya was a close ally of the United States in its global war on terrorism. But he was accused by many Mauritanians of using it as an excuse to crack down on moderate Islamist opponents.

Abdallahi, whose inauguration last month was attended by the most senior U.S. delegation ever to visit, has indicated his government will continue to cooperate with the United States.

Security officials and diplomats say al Qaeda's north African wing has been running small, mobile training camps in countries in the southern Sahara, including Mauritania, to train militants in guerrilla tactics, including handling explosives.

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