He is refusing food in protest at the life sentence he received in June for allegedly plotting against the state.
Amnesty described his trial by a military court as "grossly unfair".
His conviction was based on a confession he made under duress, and no evidence was presented showing he had used or advocated violence during the mass protests against King Hamad bin Issa al-Khalifa, it said.
'Sexual abuse and beatings' Bahrain has been wracked by unrest since pro-democracy demonstrators occupied a prominent landmark in Manama, Pearl Roundabout, in February 2011. At least 50 people, including five police officers, have been killed, hundreds have been injured and thousands jailed.
The protesters were forcibly driven out of Pearl Roundabout by security forces in March 2011, after King Hamad declared a state of emergency and brought in troops from neighbouring states to crush dissent.
Abdulhadi al-Khawaja told the BBC before his arrest on 8 April that he had deliberately stayed away from Pearl Roundabout.
"I don't want to give the authorities any reason to arrest me," he said.
He was nevertheless picked up in a late night raid and subsequently received a life sentence from a military tribunal for plotting the overthrow of the government.
According to testimony he gave to the Bahrain Independent Commission of Inquiry (BICI) - a panel of human rights experts asked to look into the unrest by King Hamad following the international outcry over his handling of the protests - Mr Khawaja suffered prolonged torture while in detention.
Mr Khawaja said his jaw had been broken in four places when police and masked men burst into his daughter's home and seized him.
He was taken to a Bahrain Defence Force (BDF) hospital and spent seven days blindfolded and handcuffed to his bed, he told the BICI. While in hospital, he and his family were threatened with sexual abuse, he said.
Amnesty's Middle East and North Africa director, Philip Luther, said on Friday: "The Bahraini authorities have made pledges that they would release people who were imprisoned for exercising their right to freedom of expression, but the continued imprisonment of Abdulhadi al-Khawaja demonstrates that they are not serious about fulfilling such promises."
According to his lawyer, Mr Khawaja has lost 16kg (35lbs) since his hunger strike began on 8 February in protest at his prison sentence.
The National Safety Court of Appeal, a military court, upheld the conviction in September, but an appeal is set to be heard by the Court of Cassation on 2 April.
The Bahraini authorities were not immediately available for comment.
Mr Khawaja, who is married with four daughters, is also a citizen of Denmark, where he lived in exile for decades. He returned to Bahrain after the government announced a general amnesty in 2001.