A Bahraini court has sentenced Zainab al-Khawaja, the daughter of a prominent imprisoned opposition activist, to one month in jail for trying to organise an anti-government protest, her mother said.

Bahrain has been in turmoil since a pro-democracy protest movement erupted last year after revolts in Egypt and Tunisia. Khawaja, the daughter of Abdulhadi al-Khawaja, who has been on hunger strike for more than three months, was arrested a month ago for trying to stage a protest in the capital Manama during Bahrain's Formula One Grand Prix.

She was fined $530 on Monday on a separate charge related to insulting a government employee.

"She was supposed to be released since she has already served the jail term," Zainab's mother, Khadija Almousawi, told the Reuters news agency by telephone from Manama.

"She refused to pay the fine and the judge [on Thursday] said she would remain in jail for [a further] 40 days unless she pays it."

Almousawi said she had visited her daughter in prison on Wednesday, and that she had been on hunger strike since Sunday in support of another imprisoned female political activist, Massouma al-Sayed, who was also on hunger strike.

"She [Zainab] had been taken to hospital four times since Sunday because her blood sugar was very low ... She looked very pale."

Bahrain's Sunni Muslim rulers have rejected calls from the mostly Shia protesters for an elected government. Violence has intensified in recent months, and protesters clash with riot police almost every day.

The authorities say the opposition are being supported and encouraged by Shia Iran, and have vowed to get tough on protests as talks with the opposition have stalled.