Pro-democracy protests in Manama centred around the now demolished Pearl roundabout monument [Reuters]

Bahrain's military prosecutor has accused 21 pro-democracy activists of seeking to overthrow the Sunni Muslim monarchy with the help of a foreign "terrorist group".

The charges, to be laid out before a military court on Sunday, are an apparent reference to Iranian-backed groups allegedly supporting the island nation's Shia Muslim majority.

The trials are part of efforts by the authorities to prosecute opposition leaders and others after months of clashes and protests in the kingdom which hosts the US Navy's 5th Fleet.

Ibrahim Sharif, a Sunni and head of the secular leftist Wa'ad Party, is set to be tried first. Others may also go on trial on Monday.

Most of the protest leaders have been in military custody at Sheikh Issa Airbase south of Manama, the capital, since 17 March, sources told Al Jazeera.

Late last month, a special security court set up under martial law sentenced four Shia men to death for killing two policemen in the unrest.

The latest cases were tried by the same court, according to the state-run Bahrain News Agency.

Fourteen members of the group are in custody, including several prominent Shia political figures. The others are charged in absentia.

The allegations include seeking to topple the 200-year-old monarchy and having links to "a terrorist organisation abroad working for a foreign country".

No addition details were made public, but Bahrain's leaders have claimed that the Iranian-backed Shia group Hezbollah in Lebanon has sought to make inroads in Bahrain with the protests.

Bahrain also is locked in a deepening quarrel with Iran, which has sharply criticised the waves of arrests and the dispatch of a 1,500-strong Saudi-led force in March to prop up the monarchy.

Protests began in February - inspired by others across the Arab world - by Shia groups demanding a greater political voice and other reforms in the tiny Gulf nation.

Shia comprise about 70 per cent of Bahrain's population, but are excluded from top government and security posts. More than 30 people have died in the unrest.