Police in Saudi Arabia have briefly detained the brother of a widely revered Shiite Muslim cleric facing a death sentence in the kingdom.

Family members say they don't know why police detained Mohammed al-Nimr on Tuesday in eastern Saudi Arabia. Reprieve, a United Kingdom-based advocacy group, also reported al-Nimr's brief detention. He has been released.

His brother, Sheikh Nimr al-Nimr, is a vocal critic of the Saudi government and a central figure in Shiite protests that erupted as part of the Arab Spring. He was sentenced to death in October last year.

Ali Mohammed al-Nimr, who is the son of Mohammed al-Nimr, is one of three young Shiite activists sentenced to death for taking part in protests before age 18. Rights groups have called for the sentences to be commuted.