Crowds gathered near the courtroom when the trial opened in February
Campaigning group Human Rights Watch has condemned Bahrain over the trial of 35 opposition activists charged with trying to overthrow the government.
The group of defendants includes Hassan Mushaima, the leader of the mainly Shia opposition movement Haq.
HRW says Bahrain is showing contempt for the right to a fair trial.
It says the televised trial uses coerced statements against the mostly young defendants who have been detained for weeks without access to lawyers.
A defence lawyer says that many of the accused at a hearing in February complained they had been tortured.
The accused also include Shia cleric Sheikh Muhammad Habib Moqdad, who with with Mr Mushaima was arrested in late January, and political activist Abd al-Jalil Singace, who has been released on bail.
The indictment said defendants set up "an illegal association that opposes Bahrain's constitution and uses terrorism as a means to achieve its goals", punishable by life imprisonment.
It also said they "propagated to overthrow the regime and the political system by force" and spread "hatred" of the government, crimes which carry between three to five years in jail.
As he entered his not-guilty plea, Mr Mushaima, 61, said the charges were "malicious", "fabricated" and "revenge" for his political activity.
Most of the defendants are young Shias said to have been trained in Syria to commit terror attacks in Bahrain, which is ruled by members of the small country's Sunni Muslim minority.