Two Bahraini women activists jailed for their alleged involvement in anti-regime protests have launched a hunger strike demanding to be released, rights group Amnesty International has said.

Roula al-Saffar, head of the Bahrain Nursing Society and Jalila al-Salman, deputy head of the Bahrain Teachers' Association, were tortured in detention and have been held for several months near the capital, the London-based group said in a statement received by AFP on Thursday.

The hunger strike "is a desperate attempt to protest against their imprisonment and the way they have been treated," said Philip Luther, Deputy Director of Amnesty International’s Middle East and North Africa Programme.

"Amnesty International is concerned that they are being held solely because they took part in protests, in which case they would both be prisoners of conscience who should be released immediately and unconditionally," he added.

Amnesty did not say when the two women began the hunger strike but said it learned that the activists started it to protest their continued detention, while others have been released on bail.

At least 500 people have been detained in Bahrain since month-long protests demanding democratic reforms broke out in the Gulf kingdom in mid-February, Amnesty International said.

Almost 2,000 people have been dismissed or suspended from their jobs, it added.

Security forces in the archipelago ruled by the Sunni Al-Khalifa dynasty cracked down on the mostly Shiite protesters in mid-March after being backed by troops that rolled in from neighbouring Sunni Gulf monarchies.