Ninety-four people, said to be members of an Islamist organisation, have gone on trial charged with plotting to overthrow the United Arab Emirates government.
The group - all Emiratis - was arrested in a series of raids last year.
The detainees include two prominent human rights lawyers, as well as judges, teachers, and student leaders.
If convicted, the group, believed to include 12 women, faces up to 15 years in jail, with no right of appeal.
The government alleges that they were part of a secret cell with links to the Muslim Brotherhood organisation.
Most of those arrested belong to the conservative religious society al Islah.
Critics say al Islah intends to replace the Emirati ruling families with a strict Islamist regime underpinned by sharia (Islamic) law, a charge human rights activists have challenged.
Nick McGeehan of Human Rights Watch (HRW) told the BBC: "We have seen no evidence in the public domain to substantiate that charge.
"As far as we are aware al Islah is a peaceful civil society that advocates a government based on more traditional Islamic precepts."
Human rights lawyers Mohammed al-Roken and Mohammed al-Mansoori are among those on trial.
Some of the defendants have been in detention for nearly a year but most were arrested in July and August 2012.
Amnesty International says a lawyer and human rights activist, Ahmad Nashmi al-Dhafeeri, who was meant to observe the trial on the organisation's behalf, was barred from entering the UAE without explanation.
The defendants, in previous court appearances extending their detention, have said they were being held in solitary confinement under strong lights 24 hours a day, seven days a week and that when they left their cells they were blindfolded.
A relative of one of the detainees, who attended one of these hearings on 6 September, 2012, told HRW that the prisoners appeared "dishevelled, disoriented, and distressed".
As well there have been claims that torture was used against some of them.
Mr McGeehan said: "I have no doubt that the allegations of mistreatment are true and we continue to have grave concerns for the well-being of all the detainees."
An Emirati human rights activist who asked not to be named said that the detainees had had no private meetings with lawyers and only limited access to their families.
He said they were brought blindfolded from "an unknown place" to the office of the state security prosecutor, where since November they have been allowed to see relatives for 30 minutes every month but only in the presence of security officers.
The activist said he had been told meetings with their lawyers took place in the same office with a representative of the prosecutor's office listening in to the conversations.
The reported circumstances of the meetings violated the confidentiality of conversations between lawyers and their clients.
"This is a clear violation of their rights. It is illegal," the activist told the BBC.
According to Mr McGeehan, by last Friday UAE authorities had still not released the identities of all 94 detainees, the documents setting out the charges against them, or the evidence on which the charges are based, despite the fact they were due in court the following Monday.
However the UAE Attorney General has insisted that the prisoners are being "dealt with according to the law"
Their trial is taking place in the Federal Supreme Court in Abu Dhabi.