Jailed Bahraini activist Abdulhadi al-Khawaja has been on hunger strike for a week seeking release, his lawyer said on Sunday, adding that authorities have arrested his daughter at the airport.

"Abdulhadi is continuing with the hunger strike he began on August 25" in Jaw prison near Manama, said lawyer Mohammed al-Jishi.

The prisoner's condition was "stable even though he suffered from hypotension two days ago," said Jishi of the last time he saw his client.

Hundreds of Shiites have been arrested and many have faced trials over their role in anti-regime protests that erupted in February 2011.

Jailed for life for plotting to overthrow the monarchy, 54-year-old Khawaja staged a 110-day hunger strike in 2012 over his imprisonment.

Bahraini authorities said they will continue to monitor Khawaja's health.

Since the start of his hunger strike, a doctor has seen Khawaja "17 times" in five days, the interior ministry's Ombudsman Office said in a statement reported by official BNA news agency.

BNA also said Khawaja sent a letter to the prison authorities saying that "he would go on a hunger strike until he is released."

Khawaja is among defendants handed lengthy jail sentences for their role in the 2011 protests. Seven of them, including Khawaja, have been jailed for life while another seven remain at large.

Amnesty International and Human Rights Watch describe Khawaja as a "prisoner of conscience."

Meanwhile, authorities late on Saturday arrested his daughter Maryam al-Khawaja -- co-director of the Gulf Centre For Human Rights which has offices in Copenhagen and Beirut -- upon her arrival at Bahrain International Airport.

She had been "stripped of her nationality," according to Jishi.

Authorities finally granted Maryam a visa but accused her of "attacking policewomen" at the airport and held her in detention "for seven days pending investigation," he told AFP.

Security forces crushed the protests in mid-March 2011, but smaller demonstrations frequently take place in Shiite villages, triggering clashes with police.

Bahrain is a strategic archipelago just across the Gulf from Iran.

Washington is a long-standing ally of the ruling Al-Khalifa dynasty, and Bahrain is home to the US Navy's Fifth Fleet.