Zainab al-Khawaja told The Associated Press that she will refuse food until her father, Abdulhadi al-Khawaja, is released, along with her husband, brother-in-law and uncle.
The 27-year-old mother of a baby girl first announced her hunger strike in a letter addressed to President Barack Obama that she posted on her blog on Monday.
The uncle was arrested in a separate police sweep while the other three men were taken into custody in a raid on Zainab's house in a Shia village outside the capital Manama on Saturday. Zainab said her father was beaten unconscious before he was taken away by armed masked men
"My father's only crime is that he has documented human rights abuses in Bahrain," Zainab al-Khawaja said. "I demand he and all men of my family are released."
Mark Toner, State Department spokesman, said the US was aware of the case and was calling on Bahraini authorities to "allow these individuals to freely express themselves and uphold their universal rights."
Authorities in Bahrain have cracked down heavily on dissent since martial law was declared last month to quell protests by the country's Shia majority against the Sunni royal family that has ruled the tiny Gulf island nation for more than 200 years.
The Shias are agitating for greater political freedoms and equal rights.
Bahrain holds particular importance to Washington as the host of US Navy's 5th Fleet, the main American counterweight to Iran's efforts to expand its military influence into the Gulf.
The United States has urged the monarchy to respect human rights but says little about allegations of repression against Bahrain's Shia.
Abdulhadi al-Khawaja, 50, is a former Middle East and North Africa director of Frontline Defenders rights organisation. He also documented human rights abuses in Bahrain for Amnesty International and Human Rights Watch.
At least 29 people have been killed since the protests began on Feb. 14, including three opposition supporters who died in custody. Hundreds of Shia activists, anti-government protesters and opposition leaders have been detained in the crackdown.
None of those in custody have been publicly charged with a crime or brought to trial.