Aktham Nuaisa was charged on Sunday with "opposing the objectives of the revolution" and "disseminating false information aiming at weakening the State" - a charge that risks a prison sentence of 15 years.
The area was sealed off by about 50 riot police as demonstrators - including many Kurds - carried posters of the defendants and banners denouncing the emergency law in force in Syria since the Baath party took power in 1963.
Heading the Committee for the Defence of Democratic Liberties and Human Rights in Syria, Nuaisa was detained in April 2004 after he organised a rare demonstration calling for the lifting of the emergency laws.
He was freed on bail after five months in detention.
No Syrian government official contacted by Aljazeera.net was prepared to comment on the trial.
Syrian speaks out
But Syrian rights lawyer and activist Anwar al-Bunni said that many international organisations have shown a great deal of interest in the case.
Speaking to Aljazeera.net after the judge adjourned the trial until 26 June, al-Bunni added that "this trial is a farce. It demonstrates clearly that the judiciary is under government control and that Nuaisa is facing a political trial".
"And what is so sad is that he is on trial for the simplest of demands. We want true democratic representation, an independent judiciary and a free press. Is that too much to ask?"
"We want true democratic representation, an independent judiciary and a free press. Is that too much to ask?"
Syrian human-rights lawyer and activist
Also on trial at the court was Kurd Shevan Abdu, who was arrested following bloody clashes in northern Syria in March 2004. Demonstrators cheered and chanted "freedom, freedom" as he arrived in court in a prison van.
The court also heard from the defence team of Massaab Hariri, who has been in detention for three years. His father is wanted for being a member of banned Islamist group, Muslim Brotherhood.
Human-rights groups have repeatedly called for the release of detainees, as well as the scrapping of the security court and the emergency law.
The Euro-Mediterranean Human Rights Network and the Observatory for the Protection of Human Rights Defenders have also voiced their deep concern over the trial.
Both commented that usual criminal procedures for a fair trial are not applicable before the Supreme State Security Court (SSSC), according to Decree 47 passed in 1968.
The SSSC does not fall under the authority of the higher judiciary but under the National Security Office of the Baath Party. And one military judge is among the panel of three judges, whose decisions cannot be appealed.
Nuaisa, international rights organisations say, has been repeatedly harassed for many years for his activities in favour of human rights in Syria.
He won the 2005 Martin Ennals Award for Human Rights Defenders.