Bahrain has announced a retrial for a hunger-striking political activist and 20 others accused of trying to overthrow the western-backed monarchy in the Gulf state's Arab spring protests last year.

Abdulhadi al-Khawaja is to be tried in a civilian court – rather than a military court as before – part of an effort by the Bahraini government to respond to domestic and international criticism of its policies by finding a face-saving solution. A senior Bahraini official suggested he might get a reduced sentence in a new trial.

But continued protest in Bahrain was backed by Amnesty International UK yesterday, saying that, pending retrial, Khawaja and 13 others should be released from custody. Human Rights Watch also called for their immediate release, saying the set-aside verdict was "mind-boggling" in its lack any specific criminal offences.

Khawaja, 52, was sentenced to life imprisonment for plotting against the state last summer. But a three-month hunger strike and an energetic campaign by family and supporters have kept his case in the spotlight. It was raised too in the runup to the recent controversial Formula One Grand Prix in Bahrain. Khawaja is in a military hospital in a serious condition, having lost 25% of his body weight; the Bahrain defence forces denied in a statement on Sunday that he was being force-fed.

The retrial is a partial victory for Khawaja, but his family insisted he must not remain in custody. "Abdulhadi al-Khawaja did not go on hunger strike saying death or retrial, he said death or freedom," his daughter Maryam wrote on Twitter. "A retrial doesn't mean much."

Khawaja's wife, Khadija al-Moussawi, told the BBC: "I think it is ridiculous. What sort of legal process is this? They are playing for time, and should have transferred his case to a civilian court at the first hearing, not the third."

Another of those convicted, Alhur al-Sumaikh, had his two-year sentence reduced to six months and was released because of time served.

The Bahrain Human Rights Society noted that the retrial would be based on interrogations by military prosecutors.

The court of cassation decision is in line with the recommendations of the Bahrain independent commission of investigation (BICI) appointed by King Hamad al-Khalifa, which found Khawaja had suffered prolonged torture while in detention. It said those convicted in the so-called "national safety courts" had to be re-tried in civil courts. But the government made clear it still viewed the case as serious. "We are talking about 21 people who called for the overthrow of the monarchy using violent means," alleged Abdulaziz bin Mubarak al-Khalifa, chief government spokesman. "In due course new evidence wil be presented in a civilian court to prove that point."

Britain's Foreign Office welcomed the decision: "We urge the courts to move this forward urgently, with due process and transparency. We call for all other upheld convictions by military courts to be reviewed without further delay."

Khawaja has dual nationality with Denmark; it ambassador criticised the decision to keep him in custody and renewed his call for Khawaja to be transferred to Denmark on humanitarian grounds; its foreign minister, Villy Soevndal, called the court decision "positive as a starting point" but "the case is not resolved yet."

John Yates, the former Metropolitan police commissioner who is acting as an adviser to the Bahraini police, had no part in the process leading to the re-trial.

A similar retrial process is under way in a civilian court for 20 medical professionals convicted by a military-led tribunal of anti-state crimes and sentenced to five to 15 years in prison. About 50 people were killed in the protests known as Bahrain's Pearl Revolution last year.

Bahrain's government, meanwhile, has been accused of urging supporters to vote in an online poll on the Radio Times website to ensure a highly critical film about last year's unrest does not win the current affairs prize at this year's Bafta Television Awards.

On Saturday, the Bahraini foreign minister, Khalid al-Khalifa, tweeted to his nearly 80,000 followers urging loyalists to vote against the al-Jazeera documentary Bahrain: Shouting in the Dark. The film has won numerous awards.

Human Rights Watch said in a new report at the weekend that Bahraini police were still beating and torturing detainees, including minors, despite the recommendations of the BICI and public commitments to end torture and police impunity. The government called the allegations "absurd" and based on "unreliable sources".

Nadim Houry, deputy Middle East director at HRW, said: "Bahrain has displaced the problem of torture and police brutality from inside police stations to the point of arrest and transfer to police stations. This abuse contradicts one of the most important recommendations of the independent commission and shows why investigations and prosecutions of abusers to the highest level are essential to stopping these practices."