A Bahraini court has sentenced 50 people to between five and 15 years in jail for setting up a group that organises anti-government protests, and that authorities say is working to topple the government by force, activists say.

Bahrain has seen almost daily protests by members of the Shia Muslim majority since February 2011, when it crushed a Shia-led uprising demanding that the Sunni Al-Khalifa dynasty give up power.

Activists said the government had accused those convicted on Sunday of membership of the February 14 movement, which has been organising protests against the government since 2011.

Bahrain's head of public prosecution had described the group as a terrorist organisation.

This was a sham trial with a political verdict, they should be released immediately Opposition activist Maryam al-Khawaja

Asked for comment, an official said a government statement on the matter was being prepared.

The main Shia opposition party Al Wefaq called it a "black day for justice".

Yousif al-Muhafda from the Bahrain Centre for Human Rights said that "a group of February 14 activists were sentenced to between five and 15 years in jail".

The group said there were human rights campaigners among those convicted "under the internationally criticised and vague terrorism law", and that the sentences added up to more than 400 years in jail.

"This was a sham trial with a political verdict, they should be released immediately," the group's acting president, Maryam Al-Khawaja, said in a statement.

Mohammed al-Maskati, head of the Bahrain Youth Society for Human Rights, which describes itself as a local rights group, said a member of the society's board had been given a 15-year sentence.

Some of the suspects were convicted in absentia.

Bahrain's Shia Muslims have long complained of entrenched discrimination in areas such as employment and public services, allegations that the Sunni-led government denies.

The persistent unrest has placed Bahrain on the front line of a struggle for regional influence between Sunni Saudi Arabia, Bahrain's close ally, and Shia Iran, which denies Bahraini accusations of fomenting Shia protests.