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Ebtisam Al-Saegh had retweeted on Monday criticising security forces' treatment of women and blaming the king.

She was arrested at her home on Monday night when her house was raided by masked officers. She was then taken to a prison in Manama, Amnesty said.

"We are deeply concerned about Ebtisam's wellbeing. When she was arrested in May 2017, she was beaten and sexually assaulted by members of the Bahraini National Security Agency," said Samah Hadid, director for campaigns at Amnesty International.

"Bahraini authorities have failed to investigate those claims and we fear that she is at high risk of torture as long as she remains in custody."

Amnesty said no warrant was present at the arrest.

The tiny Gulf kingdom has been rocked by persistent unrest since its Sunni minority rulers crushed Shia-led protests in 2011 demanding a constitutional monarchy and an elected prime minister.

Authorities have detained and tried hundreds of Shias and cracked down hard on opposition groups despite appeals by international rights groups.

Last month, they banned Bahrain's main secular opposition group The National Democratic Action Society and froze all its assets.