Amnesty International has welcomed the release of 23 opposition activists in Bahrain, but called for an independent investigation into claims that some of them were tortured while in custody.

The 23 men were among at least 250 detainees released early yesterday by order of Bahrain's head of state, King Hamad bin Isa al-Khalifa, apparently in response to demands made by protesters seeking political reform in the country.

The 23 were facing trial on an array of security-related charges, which they denied but which could have resulted in their being sentenced to death.

Amnesty International Middle East and North Africa Director Malcolm Smart said:

"While we welcome the release of these opposition activists, we continue to urge the Bahraini authorities to conduct a thorough, independent investigation into allegations that some of them were tortured in pre-trial detention, and to bring to justice anyone found responsible for torture.”

The 23 were arrested in August and September last year during a clampdown in the run up to parliamentary elections held in October 2010. They were charged with forming an illegal organisation, aiming to overthrow the government and dissolve the Constitution, inciting people to "overthrow and change the political system of the country", fundraising and planning terrorist acts and other offences under Bahrain's 2006 anti-terrorism law.

According to a lawyer for the group, Mohammed al-Tajer, it is not clear whether they were released yesterday under royal pardons or if the cases against them can be reinstated at a later date.

Two others who were charged together with the 23 (but in their absence as they reside in London) - Hassain Meshaima’, secretary general of the unauthorised opposition organisation, al-Haq, and Sa’eed al-Shehabi, secretary general of the Bahrain Freedom Islamic Movement - are also reported to have had the charges against them withdrawn under a pardon issued by the King.