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Hundreds of Bahrainis who have had their citizenships revoked by courts following anti-government protests and unrest have had their nationality restored, local media have reported. 

King Hamad ordered the citizenship of 551 Bahrainis reinstated, official media said Sunday, which comes days after the UN expressed concern about the punishment.

King Hamad also requested authorities take into account "the nature of crimes committed", the Bahrain News Agency reported with the ministry ordered to examine their files and prepare lists of those to be freed.

Around 990 Bahrainis - mostly part of the Shia majority - have had their citizenship revoked since protests ignited against the Sunni ruling family in 2011.

A mass trial on Tuesday saw a Bahraini court revoke the citizenship of 138 people and jailed 139, who were convicted of belonging to a "terror" group with links to Iran's Revolutionary Guards.

It was the biggest loss of citizenship in Bahrain in one mass trial since 2012, according to the Bahrain Institute for Rights and Democracy.

The UN High Commissioner for Human Rights on Thursday expressed alarm about the court's decision. 

"Tuesday's convictions give rise to serious concerns about the application of the law, particularly through a mass trial that reportedly lacked the procedural safeguards necessary to ensure a fair trial," rights commissioner Michelle Bachelet said in a statement.

"Deprivation of nationality must not be arbitrary, especially on discriminatory grounds," she said, stressing that "arbitrary deprivation of nationality places the individuals concerned and their family members in a situation of increased vulnerability to human rights violations."