The surprise royal order gave no explanation for King Hamad bin Isa Al Khalifa's decision, other than to say that he had the final authority in such cases.
Bahrain, which is home to U.S. and British naval bases, faces widespread international criticism over its continuing crackdown. The Sunni monarchy crushed an Arab Spring uprising in 2011 supported by the nation's Shiite majority.
"The study and evaluation of the situation of convicts should be based on criteria pertaining to the seriousness, impact and consequences of the crimes, as well as on the danger the convict may pose on national security," the state-run Bahrain News Agency said in announcing the king's decision.
Authorities later will announce the names of those having their citizenship restored.
The tiny island off the coast of Saudi Arabia has seen protests, unrest and militant attacks on police since its 2011 protests, which authorities have blamed on Iran. Iran long has denied being involved.
Bahrain has targeted journalists, activists, Shiite religious leaders and political parties. Some activists have escaped into exile while others have been imprisoned.
Last week, 138 people lost their citizenship in a mass trial. That drew a rebuke from U.N. High Commissioner for Human Rights Michelle Bachelet, who described the convictions as giving "rise to serious concerns" about the country's legal system.
The Bahrain Institute for Rights and Democracy said last week's verdict brought to 990 the number of people ordered stripped of their nationality since 2012.
Sayed Ahmed AlWadaei, the director of advocacy at the institute, said he was surprised by the news. However, he cautioned that those like himself who had their citizenship stripped at the ministerial level, rather than through the courts, likely wouldn't benefit from the king's order.
"I honestly think there is something going on behind scenes, maybe some diplomatic pressure is applied to the government," AlWadaei said. "There must be a state behind it, maybe Britain or the United States."
Bahrain remains a crucial part of American military strategy in the Persian Gulf by hosting the U.S. Navy's 5th Fleet. The Trump administration has approved a multibillion-dollar sale of F-16 fighter jets to Bahrain without the human rights conditions imposed by the State Department under President Barack Obama. Activists say the Americans have eased pressure on Manama and its Gulf Arab allies.
The U.S. State Department and the British Foreign Office did not immediately respond to requests for comment.
King Hamad in the past has granted pardons to those convicted amid past crackdowns.
"Should those individuals really go through this experience?" AlWadaei asked. "The truth is he is the person behind all these repressive laws."
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