A Bahraini woman and her son were sentenced on Monday to three years in prison for planting a "fake bomb", activists said.

Sayed Ahmed Alwadaei, head of advocacy at the Bahrain Institute for Rights and Democracy (BIRD), said his relatives were convicted based on confessions gained by torture and said the court was persecuting the family for his activism.

Alwadaei's mother-in-law Hajer Mansoor and brother-in-law Sayed Nizar Alwadaei were each sentenced to three years in jail for planting a "fake bomb," BIRD said, while his cousin Mahmood Marzooq was sentenced to a month and half for obtaining a knife.

All three have been in custody since March and were not in court for the sentencing.

The case has come under international scrutiny over both the validity of the charges and the trial procedures.

Mr Alwadaei, who is based in Britain, said he believed the case against his relatives was motivated by his human rights work and especially a protest last year in which he took part against the attendance of Bahrain's king at a royal horse show in Britain.

Human Rights Watch condemned Monday's verdicts.

"The judicial process was marred by due process violations and allegations of ill treatment and coerced confessions," said the New York-based rights watchdog, adding that an appeal would be filed.