A group of 38 women and seven girls were apprehended without lawyers present and some of them reportedly tortured or otherwise ill-treated," the London-based advocacy group Amnesty International said in a statement late on Monday.
The Bahraini authorities have "patently denied these women and girls their rights after rounding them up at a Manama shopping centre," said Philip Luther, Amnesty's deputy director for the Middle East and North Africa.
Luther also called on the Bahraini authorities to allow the detainees the basic rights of access to their lawyers and visits with their families.
The women's lawyers, who had a brief encounter with a few of the detainees in the hallways of the Public Prosecution Office, told Amnesty some of them bore visible signs of bruising.
Nour al-Ghasla, 20, one of the detainees, "had bruises on her face apparently from ill-treatment in custody," Amnesty said.
Bahrain's main Shiite opposition group, Al-Wefaq, said Monday in a statement that the seven girls were "minors aged between 12 and 15 years" and they too were "beaten and humiliated."
The women were arrested on Friday, a day before by-elections boycotted by the opposition to replace 18 Shiite MPs who quit parliament in protest over violence used against pro-democracy protesters in February.
According to Amnesty, the girls remain in custody despite a Bahraini Juvenile court order to release them.
But a Bahraini human rights group said 23 of the detained women had been released late on Monday, including the seven minors.
"The seven children were among 23 detainees released Monday," Mohammed al-Maskati, head of the Bahrain Youth Society for Human Rights told AFP Tuesday, adding 15 detainees remained in custody.
Their trial, which was set to take place Tuesday, was postponed until 29 September, Maskati added.
The official Bahraini news agency Monday night announced the trial of 23 people arrested at the City Centre shopping mall last Friday.
The statement said the detainees "stand accused of unlicenced rallying, public incitement on regime-hatred and taking part in illegal demonstrations," but did not specify if the trial was for the women arrested that day.