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.

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.  

Hassiba Hadj Sahraoui, a regional deputy director at Amnesty International, said in a statement earlier:

Khawaja's arrest ... demonstrates the authorities' utter contempt of freedom of expression and peaceful protest.

After her release she tweeted that the last few days had been "tough".

Read our news story for more detail and context: Bahraini blogger released on bail