* 21 defendants charged with trying to topple government
* Rights groups say military trials not fair
MANAMA, May 8 (Reuters) - Bahrain put 21 mostly Shi'ite activists, including a prominent hardline dissident, on trial on Sunday, charged with trying to topple the government during weeks of protests in February and March.
Bahrain, a Sunni-ruled island kingdom, cracked down on the protests demanding greater political freedoms, a constitutional monarchy and an end to sectarian discrimination.
The crackdown, in which neighbouring Sunni-led Gulf states sent troops to back Bahrain's forces, has boosted regional tension with Iran, which Bahrain accuses of manipulating its Shi'ite co-religionists to expand its influence.
Those on trial on Sunday face a hybrid civilian-military court where military prosecutors try the case before a panel of one military and two civilian judges.
Those on trial include Shi'ite dissident Hassan Mushaimaa, leader of the opposition group Haq who has called for the overthrow of the Sunni al-Khalifa monarchy, and Ebrahim Shareef, the Sunni leader of the secular Waad group that has called for a constitutional monarchy but has not joined those seeking to oust the king.
Bahrain's state news agency said the defendants were accused of involvement in an "attempt to overthrow the government by force and in liaison with a terrorist organisation working for a foreign country".
Rights groups said the defendants should be tried before civil courts, saying the military courts did not allow the accused to defend themselves properly.
"Lawyers were called less than 24 hours before the trial started. They did not have time to prepare," Nabeel Rajab of the Bahrain Centre for Human Rights told Reuters.
State media said the men were accused of organising and managing a terrorist group bent on toppling the government and corresponding with a terrorist group abroad working for a foreign country against Bahrain, among other charges.
The state news agency did not say what penalty military prosecutors were seeking in the case. Rights activists say Bahrain's anti-terrorism law could make them eligible for the death penalty if convicted on the most serious counts. Of the 21 defendants, seven were being tried in absentia.
International human rights groups and Western nations have criticised Bahrain's military trials in the wake of the unrest by mostly Shi'ite protesters.
A Bahraini court last month sentenced four men to death after they were convicted in a brief trial of killing two policemen by running over them in their cars.
New York-based Human Rights Watch said on Wednesday it had received credible reports that one of the 21 defendants, Abdulhadi al-Khawaja, had been hospitalised after beatings while in custody.
Bahrain denies there is torture in its prisons and says all such accusations will be investigated.
Minister for Justice and Islamic Affairs Sheikh Khaled bin Ali al-Khalifa rejected the criticism and said even under Bahrain's current martial law all suspects would get fair trials and would have the right to appeal verdicts.
"Bahrain is keen to honour its international obligations and respect human rights covenants," he said in remarks carried by pro-government media on Sunday. (Manama newsroom; +973 1750 2031; Editing by Cynthia Johnston)
Our Standards: The Thomson Reuters Trust Principles.