SAO TOME, Oct 10 (Reuters) - Members of Sao Tome and Principe's elite police corps, known as the "Ninjas", who seized the West African state's police headquarters two days ago have freed some 20 captive officers, their leader said on Wednesday.

The Angolan-trained commandos released the policemen late on Tuesday after President Fradique de Menezes' government agreed to negotiate over their demands for a bonus and their own headquarters.

"The (police) commander and all the officials are now free to carry out their duties," Wilson Quaresma, spokesman for the Ninjas, told Reuters, adding that his men would remain in the police offices until the conclusion of the talks.

The tiny archipelago's colonial capital Sao Tome was calm. It was the second time in less than a year the Ninjas had staged a pay protest in the impoverished former Portuguese colony of some 170,000 people.

The Ninjas say they were promised a bonus payment during their training in Angola in 2003-2004, something the government denies.

Sao Tome, which lies at the centre of the oil-rich Gulf of Guinea, has been touted as the newest member of sub-Saharan Africa's growing club of petroleum exporters, but any production remains years in the future.

U.S. oil major Chevron Corp raised industry hopes when it announced an oil and gas discovery in the joint-development zone between Sao Tome and Nigeria. Tests this year should indicate if the discovery is commercially viable.

The discovery of large petroleum deposits in neighbouring Equatorial Guinea in the 1990s led to a series of attempted coups.

The Portuguese-speaking country continues to grapple with biting poverty. It won $327 million in debt relief in February from the World Bank and International Monetary Fund, but faces enormous challenges to provide basic health care and education.

With the United States looking to improve security in the Gulf of Guinea, which supplies nearly a fifth of its oil imports, the U.S. navy selected Sao Tome as the site for an $18 million radar listening post to monitor shipping in the region.

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