Images of Zainab al-Khawaja's arrest had received wide coverage on media and the internet [Reuters]

A daughter of a prominent Bahraini activist who was detained last week during an anti-government demonstration has been freed after spending five days in custody.

Zainab al-Khawaja, herself an activist and a well-known blogger, was released pending trial late on Tuesday, along with another activist Masoma al-Sayyid, Bahrain's government said.

The arrest of both women at a protest on Thursday drew criticism from rights groups.

"An investigation has been opened to review the arrest and legal procedures relating to the two women," the government said in a statement.

Shocking arrest

Images of a policewoman dragging Khawaja on the ground by the handcuffs she had placed on the blogger had received wide coverage on media and the internet.

"Khawaja's arrest ... demonstrates the authorities' utter contempt of freedom of expression and peaceful protest," Hassiba Hadj Sahraoui, a regional deputy director at Amnesty International, said in a statement earlier.

Khawaja, 28, is the daughter of Bahrain's most prominent political activist, Abdulhadi al-Khawaja. He was imprisoned for life along with seven other opposition leaders in June.

A special security tribunal, set up under emergency rule, convicted them of anti-state crimes.

Along with her father, Khawaja's three other male relatives and her husband have been imprisoned since February, when the unrest between Bahrain's Sunni monarchy and an opposition movement led by the country's majority Shia started.

Pending US arms deal

Washington has said a pending $53m arms sale to Bahrain will hinge partly on the monarchy halting the abuses inflicted on protesters outlined in a report by a government-appointed fact-finding commission of international lawyers.

Separately, authorities said on Tuesday they would reinstate the last group of state employees suspended during protests mostly by majority Shia Muslims earlier this year.

"In accordance with the recommendations made by [the commission], an order reinstating the remaining [180] suspended employees to their jobs in the public sector has been issued," an official statement said.

Inspired by "Arab Spring" uprisings in Tunisia and Egypt, thousands of mainly Shia Bahrainis took to the streets in February and March demanding curbs on the power of the ruling Al-Khalifa family and an end to perceived discrimination.

The broader pro-democracy movement was suppressed with the help of military forces brought in from Saudi Arabia and other Gulf nations.