RIYADH, Sept 22 (Reuters) - One of 10 reformers jailed in February has been freed after a series of petitions for their release, but another activist is on trial in a different case.

Essam al-Basrawi was released on Thursday, his lawyer Bassem Alem said. He said it was not clear why the authorities, who could not immediately be reached for comment, let Basrawi go.

He was one of 10 men detained in February for allegedly collecting donations and giving them to "suspicious elements".

Government officials later suggested their actions involved helping insurgents in Iraq.But no formal charges were laid, and lawyers and colleagues of the men say the arrests were in fact designed to scupper plans to set up a political party.

Saudi Arabia has no parties or elections to its parliament, and political life is dominated by the ruling Al Saud family.

Basrawi's release came after three petitions in the space of a month.

"Far too much time has passed since they were detained, enough time to establish their innocence or indict them to face a fair and public trial," a statement by 70 Saudi rights activists said last week, claiming some of the men had begun a hunger strike. "Even lawyers have not been allowed to see them."

In a separate case, the third court session in the trial of well-known reform activist Abdullah al-Hamed and his brother Isa took place last Wednesday, their lawyers told Reuters.

The trial has been held behind closed doors in the town of Buraida, north of Riyadh.

The lawyers said in a statement that prosecutors accused Hamed of encouraging the wives of political detainees to stage public protests about their situation.

The government says about 3,000 people are in detention out of a total of 9,000 arrested since Islamist militants allied to al Qaeda launched a violent campaign in May 2003 to topple the U.S.-allied monarchy and expel foreigners.

Human rights activists and reform advocates say many of these detainees have little or no connection to militant groups and accuse the Interior Ministry of repressing dissent.

King Abdullah is viewed by many Saudis as a supporter of some political reforms but diplomats say his room for manoeuvre is restricted by opposition from powerful members of the royal family. His half-brother Prince Nayef is Interior Minister.

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