King Hamad Al-Khalifa on Sunday ordered that the state of emergency will be lifted at the end of the month.
The trial of the 21 - seven of whom are abroad and to be tried in absentia - was adjounred until Thursday.
Only lawyers and two members of the family of each defendant were allowed to attend the trial, according to the Bahrain Centre for Human Rights. Lawyers are banned from talking to the media, the organisation said.
The group includes both Sunni and Shia Muslims – the majority of the counrty’s population is Shia, while the ruling Khalifa family is Sunni.
They are charged with "forming and directing a terrorist group aimed at overthrowing and changing the constitution of the state and the monarchy system," the official news agency, BNA, said.
They are also accused of "having contact with a terrorist group abroad that operates in the interest of a foreign country by conducting hostile acts against the kingdom of Bahrain" and
"raising funds for the terrorist group despite knowing" the nature of the organisation.
A military court last month sentenced four protesters to death and three others to life in jail after finding them guilty of killing two police officers. Last week another was sentenced to seven years after being found guilty of attempting to murder a police officer.