A Bahraini protester wearing a shirt showing jailed human rights activist Abdulhadi al-Khawaja takes part in an anti-government protest on September 5, 2014 west of Manama

A prominent Bahraini activist jailed for life for his role in anti-regime protests has been on hunger strike for two weeks to protest the treatment of prisoners, campaigners said Thursday.

Abdulhadi al-Khawaja, who also holds Danish citizenship, is an outspoken critic of Bahrain's Al-Khalifa monarchy, which has ruled the tiny Gulf archipelago for more than two centuries.

Khawaja began the hunger strike on April 12 in a Manama prison to protest the "arbitrary detention and degrading treatment of prisoners in Bahrain," the Gulf Centre for Human Rights (GCHR) said in a statement.

It is the fifth such protest in six years by Khawaja, who was given a life sentence in June 2011 after being convicted of plotting to topple the monarchy.

Sunni-ruled Bahrain, home to a majority Shiite population, has witnessed sporadic unrest since early 2011, when long-running demands for political reform intensified with nationwide demonstrations calling for an elected government.

Authorities have since tightened their grip on Shiite-led dissent, jailing hundreds of protesters and stripping activists and clerics of citizenship.

The GCHR also called for the medical evacuation of Khawaja and fellow rights activist Nabeel Rajab, who is behind bars while on trial for charges of insulting the state.

Rajab, one of the most high-profile activists in Bahrain, underwent minor surgery earlier this month. His family has however said they remain concerned about his health.

The UN is scheduled to hold a human rights review session on Bahrain in Geneva on May 1.