RIYADH, Aug 28 (Reuters) - Authorities in Saudi Arabia, the world's biggest oil exporter, have released a leading reform activist after he served a six-month prison sentence, acquaintances said on Thursday.

Abdullah al-Hamed, a leading intellectual figure among Islamist reformists, was freed early on Thursday, preacher Mohsen al-Awajy and rights activist Fawziya al-Oyouni told Reuters.

Hamed's sentence was widely seen as punishment for his calls for political reform in the conservative Islamic monarchy, a key U.S. ally.

He was jailed by a Saudi court for encouraging the wives of suspected Islamist militants to publicly protest their husband's indefinite detention.

"I hope Hamed's release will be the beginning of a new phase and that this will be a good sign regarding the status of reformers," said Islamist activist Khaled al-Omair.

Saudi intellectuals this week urged the government to do more to stop detentions and other restrictions placed on figures, both liberals and Islamists, who work publicly to turn the absolute monarchy into a parliamentary democracy.

The Interior Ministry has detained several thousand people since al Qaeda-linked militants launched a campaign of violence in 2003 to destabilise the Al Saud family's rule. But critics say the crackdown has extended to lobbyists for peaceful reforms. (Reporting by Andrew Hammond; editing by Dina Kyriakidou)

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