Iran's parliament is in a power struggle with the judiciary
Outside the court more than 20 relatives of those on trial staged a sit-down protest in sub-zero temperatures, claiming they were prevented from entering the court.
Narges Mohammadi, the wife of accused activist and writer Taqi Rahmani, told the Associated Press news agency that there was "nothing valid in the trial".
And defence lawyer Mohammad Ali Dadkhah said that he also had been barred from attending despite representing two of those accused.
Another defence lawyer, Mohammad Ali Jedari-Forughi, resigned from the case, saying he had still not been officially informed of the charges against his clients, nor been allowed to read their files.
Most of the defendants are writers and university professors from the liberal Religious-Nationalist Alliance.
They face charges of trying to provoke student unrest, having secret ties with foreigners and conspiring to overthrow the Islamic state.
"The trial is largely political rather than legal, this explains the decision to hold it behind closed doors," he said.
Protest
Iranian reformers, international human rights organisations, the dissidents and their families had already sent a open letter to the UN's High Commissioner for Human Rights, protesting the decision to hold the hearing in private.
"This is another flagrant violation of the prisoners' basic rights," the families said.
"We are afraid they will be denied the benefit of a jury in a closed trial."
The defendants include veteran opposition leaders Ezatollah Sahabi and Habibollah Payman, academic Reza Raiss-Tousi and journalists Taqi Rahmani and Hoda Saber.
The accused were among dozens of people arrested by police in a series of raids last year. Six of them are being held in a military detention centre. The rest are free on bail.
The case has heightened tensions between Iran's hard-line judiciary and its reformist-dominated parliament.
Mehdi Karroubi, the assembly's moderate speaker, has called for the dissidents' release. And more than half the parliament's deputies have questioned the methods used to extract confessions.
Power struggle
The BBC's Eurasia analyst says the judiciary is widely seen as a stronghold of entrenched hardliners opposed to the reforms favoured by embattled President Mohammad Khatami and his supporters in parliament.
Last month, a court jailed a reformist deputy for insulting the judiciary. Some 60 other members of parliament are also currently being prosecuted.
But as the power struggle continues, one senior Iranian cleric has stood up in support of the country's parliament.
On Monday cleric Ayatollah Yousef Sanei said parliament was the most important institution in Iran and all other government bodies should respect its special status.
Ayatollah Sanei was speaking to a group of deputies visiting his home in the holy city of Qom to seek support for parliament.
Public opinion
Among members of the Iranian public, there is growing unease over the way the courts are dispensing justice.
Five months ago a riot broke out when protesters and police clashed at a public hanging.
Public executions are still commonplace in Iran. On Sunday, three men were publicly hanged. One had been found guilty of murder, the other two of rape.
Iran's courts uphold Islamic Sharia law, according to which the punishment for murderers and rapists is execution.