Bahrain's king has reinstated the citizenship of 551 people convicted by courts during a crackdown on dissent.
No reason was given for the decision and those affected were not named.
At least 990 Bahrainis have reportedly lost their citizenship through court decisions or executive orders since 2012. Most have been left effectively stateless, and some have been deported.
They have included many human rights defenders, political activists, journalists and religious scholars.
Last week, a court sentenced 139 men to between three years and life in prison, and stripped all but one of them of their citizenship, after finding them guilty of terrorism charges.
Prosecutors alleged that they set up a militant group linked to Iran's Islamic Revolution Guards Corps (IRGC), detonated bombs and damaged property.
The UN High Commissioner for Human Rights, Michelle Bachelet, expressed alarm at the court decision, saying she had "serious concerns that the proceedings failed to comply with international fair trial standards".
A large number of the accused were reportedly tried in absentia, and 17 of those convicted were believed to be minors aged between 15 and 17, she added.
Ms Bachelet also warned that the revocation of nationality could have serious consequences for the human rights of the individuals concerned and their families in all aspects of their daily lives, including the denial of the right to health, education and freedom of movement.
"Under international law, revocation of nationality is prohibited if it does not serve a legitimate aim or is disproportionate," she said.
Article 15 of the Universal Declaration of Human Rights states that "everyone has the right to a nationality" and "no-one shall be arbitrarily deprived of his nationality".
The Sunni-ruled Gulf island has been wracked by unrest since security forces crushed pro-democracy protests led by the Shia majority community in 2011.
The authorities have accused Iran of backing militants who have carried out attacks on security forces. Iran has repeatedly denied the charge.