The Bahrain Center for Human Rights said Abdulhadi al-Khawaja, who formerly worked for international human rights organizations, was detained on Saturday in a pre-dawn raid.
Al-Khawaja's daughter, Zainab, confirmed the arrest and said her father was taken from her house in a Shiite village outside the capital, Manama.
She told The Associated Press that armed and masked men, some wearing black police uniforms and carrying riot gear, stormed her house around 2:00 a.m. on Saturday. They beat her father unconscious before leading him into custody along with her husband and her brother-in-law, she added.
"They were not just slapping him around, they were beating him badly like they wanted to hurt him," Zainab al-Khawaja said on the phone. She said one agent was holding her father by the neck and at least four were beating him severely and kicking him as they were dragging him down a flight of stairs.
"They kept saying to him 'We will kill you' and I begged them to not beat him because he is willing to go with them peacefully," the activist's daughter said. "I heard my father gasping for air, saying he cannot breath, but they just kept hitting him until he passed out." Al-Khawaja, 50, is a former Middle East and North Africa director of Frontline Defenders rights organization. He also documented human rights abuses in Bahrain for Amnesty International and Human Rights Watch. His daughter said he stopped working for international organizations last year because of harassment by the authorities.
Bahrain declared emergency rule last month and cracked down on protests by the country's Shiite majority against a Sunni monarchy, detaining hundreds of activists and anti-government protesters. At least 27 people have been killed since protests in the strategically important Gulf kingdom began on Feb. 14.
Bahrain is the home of the U.S. Navy's 5th Fleet.