"Ongoing violations of the rights to freedom of expression, peaceful assembly and association, and the targeting of human rights activists in Bahrain remain of serious concern," Ravina Shamdasani, the spokeswoman for the office of the UN High Commissioner for Human Rights said in a statement.
Maryam al-Khawaja, the daughter of a jailed opposition figure, was arrested on August 30 as she flew into the country to visit her father.
Shamdasani said the UN agency was "seriously concerned" that Al-Khawaja, who is the co-director of the Gulf Centre for Human Rights, was arrested for her activism.
She has since been detained in a women's prison on charges of assaulting a police officer and is due to appear in court this Saturday.
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The spokeswoman said that her office "has been in touch with the authorities to raise our concerns on this case," as it called for her immediate release.
"We urge the government to take immediate steps to release Ms. Al-Khawaja and all human rights defenders and individuals detained for the peaceful exercise of their rights, and to ensure that all human rights defenders in Bahrain are able to carry out their important work in an effective manner without fear of harassment or prosecution," the spokeswoman said.
The spokesperson also referenced strong sentences against a photojournalist and another activist upheld after recent appeals, and the "hundreds of young people... Still in detention or serving lengthy sentences for their participation in demonstrations."
The tiny but strategic Bahrain, just across the Gulf from Iran and home base for the US Fifth Fleet, remains deeply divided three years after authorities crushed the uprising with Saudi-led military backing.