The rights group accuses Bahraini authorities of mistreating activist Abudlhadi al-Khawaji while he was in custody on charges of attempting to topple the minority Sunni regime.
His family members, Human Rights Watch said, reportedly observed signs of torture and fractures on his face that needed reconstructive surgery.
"It appears that Abdulhadi al-Khawaja's jailers tortured him during the month they held him in incommunicado detention," Joe Stork, deputy Middle East director at Human Rights Watch, said in a statement Tuesday. "Torture or ill-treatment is a serious crime, and Bahraini officials who did or authorized this treatment need to be held accountable."
The rights group said it had documented evidence of similar security concerns in Bahrain.
Two members of an outlawed Shiite al-Wefaq political party, Jawad Ferooz and Mattar Ibrahim Mattar, were arrested last week in what Human Rights Watch said was the first series of arrests targeting elected officials.
The government banned the Shiite party and the opposition Islamic Action Society in April for breaking the law during mass protests and for inciting violence.
Wefaq in 2009 took 18 seats in the 40-member Parliament.