U.S.-based group Human Rights Watch urged Bahrain on Wednesday to investigate allegations by four opposition activists that they were tortured in detention last month.

Bahrain has banned media from reporting on the case of Abduljalil Singace, head of human rights activities at the mainly Shi'ite Haq movement, and other opposition members.

Their detention on security grounds sparked widespread protests, some violent, in the small Gulf Arab kingdom where parliamentary elections are due on Oct. 23.

"Bahraini authorities should immediately investigate these allegations of torture and guarantee the physical and psychological well-being of the four men," Joe Stork, deputy Middle East director at HRW, said in a statement.

He called on authorities to throw out any coerced confessions and any evidence obtained by ill-treatment.

"According to people familiar with the proceedings, Singace, who had spent the previous 15 days in incommunicado detention, told (the attorney general) of having been handcuffed and blindfolded the entire time ... (and) that his captors beat him on his fingers with a hard instrument, slapped him around, and pulled and twisted his nipples and ears with tongs," HRW said.

Other detainees complained of similar treatment, it said, adding that prosecutors had charged the four men with various national security and counterterrorism crimes and ordered another 60 days of detention.

Bahrain, home to the U.S. Navy's Fifth Fleet, is governed by a Sunni ruling family but has a majority Shi'ite population that complains of discrimination in jobs and services. The government denies this accusation.

Around 20 people have been arrested in the latest round of detentions, according to official figures, but Nabeel Rajab, head of the Bahrain Centre for Human Rights, said last week his group had registered between 40 and 60 detainees.

Singace was detained upon his return from London where he gave a lecture on human rights in Bahrain. He and another activist arrested last month, Mohammed Magdad, were held for several months in 2009 on charges of plotting to overthrow the government.

They were later pardoned by the king after weeks of protests and criticism from human rights organisations.

Night-time clashes between security forces and young Shi'ite protesters burning tyres and throwing petrol bombs are a frequent occurrence in Bahrain, a regional banking centre.

Diplomats say the arrests appeared to be an effort to press the opposition to temper protests before the elections.

Bahrain has the only elected parliament in the Gulf Arab region apart from Kuwait, although bills need to be approved by an upper house whose members are appointed by the king.

(Source: Reuters)