Mohammed al-Tajer was arrested at his home on Friday by "more than two dozen uniformed and plainclothes security officers, most of whom were masked," Human Rights Watch HRW said in a statement on Sunday, adding that no reason was given for his arrest.
"Human Rights Watch believes that Tajer is the first defence lawyer detained in more than a decade. He is well known for defending opposition figures and rights activists arrested in security sweeps," it said.
"The government’s arrest of a leading defence lawyer shows that Bahrain is taking a turn for the worse on human rights," Joe Stork, HRW's deputy Middle East director, said in the statement.
"The authorities should either release Mohammed al-Tajer or charge him now with a recognisable offence."
HRW also said that Dr Sadeq Abdullah, a surgeon at the Salmaniya Medical Centre, was summoned to the interior ministry on April 14 but not yet returned home. He later called his wife to tell her he was "fine."
Family members were allowed to drop off his medications but neither they nor his lawyer have been authorised to see him, HRW added.
"Abdullah is one of at least 19 doctors arrested by authorities since March 17, at least eight of whom were arrested within the past week," HRW said.
"We have serious concerns regarding the well-being and safety of some of the detainees," Stork said. "The authorities should immediately provide information on the whereabouts of all detainees arrested since March 17."
Protests calling for political reform broke out on February 14 in the Sunni-ruled, Shiite-majority Gulf state. Saudi-led Gulf forces entered the country in late March, freeing up Bahraini troops to crush the uprising.