DUBAI (Reuters) - A court in Bahrain revoked the citizenship of 115 people and gave 53 of them life sentences on terrorism charges, the public prosecutor said on Tuesday, in one of the most severe rulings yet in the Gulf island kingdom.

Western-backed Bahrain, where the U.S. Fifth Fleet is based, has been cracking down on Shi’ite opposition groups and rights activists since they led pro-democracy Arab Spring protests in 2011, a crackdown that has been condemned internationally.

Authorities in Sunni Muslim-ruled Bahrain have accused scores of people of militancy in a series of mass trials, saying the defendants are backed by Shi’ite Iran. Rights activists say they have included mostly peaceful opposition members.

This case pertains to Bahrain’s allegation in 2015 that Iranian Revolutionary Guards helped Shi’ite Bahraini fugitives join forces to set up a militant group called the Zulfiqar Brigades to destabilize the kingdom.

In a statement, prosecutors said 138 people had been accused of “being behind a number of explosions, possession of explosives and training in the use of weapons and explosives and the attempted murder of police officers”.

Fifty-three defendants were sentenced to life terms and 62 others to between three and 15 years in prison, while 23 were acquitted, they said.

Bahraini activists say members of the Shi’ite majority are subjected to systematic political and economic discrimination by the government, a charge the authorities deny.

Since 2011, demonstrators have clashed frequently with security forces, who have been targeted by several bomb attacks.

Bahrain has accused the opposition of undermining security and blamed the bombings on Iran and Lebanon’s armed Shi’ite group Hezbollah. Iran and Hezbollah deny any involvement in Bahrain’s unrest.

“This outrageously harsh sentence is setting a new level of injustice in Bahrain. Rendering people stateless in a mass trial is a clear violation of international law. This is the worst verdict on the record”, said Sayed Ahmed Alwadaei, an activist with London-based Bahrain Institute for Rights and Democracy.

The latest ruling brought to 717 the number of Bahrainis stripped of their citizenship since 2012, he said in a statement, adding that there had been 213 cases this year alone.

A Bahraini court in January sentenced two people to death and 19 to life in prison on terrorism charges while stripping citizenship from 47, saying they were part of a terrorist cell trained in the use of heavy weapons and explosives.