The suspects include judges, lawyers, academics and student leaders.
The attorney general alleges they formed a "secret society" with links to the Muslim Brotherhood that plotted to overthrow the government.
Human rights groups say the trial is deeply flawed and have called it a "mockery of justice".
Political parties and demonstrations are banned in the UAE, which comprises seven sheikdoms run by ruling families.
Several of its Gulf neighbours - including Bahrain, Yemen, Oman and Saudi Arabia - have seen pro-democracy protests inspired by the revolutions in Tunisia and Egypt in 2011.
The activists were arrested last year during a campaign against civil society activists suspected of political dissent.
Many are connected to the Islamist group al-Islah, which the authorities say has links to the Muslim Brotherhood.
Al-Islah says it favours peaceful reform and denies links to the brotherhood.
They are accused of using the media and social networking sites to try to turn people against the UAE's system of government and ruling families.
"They launched, established and ran an organisation seeking to oppose the basic principles of the UAE system of governance and to seize power," the attorney general said.
Human rights groups say the trial is a sham.
They say the defendants were denied access to lawyers until the last two weeks before trial, and have not been shown documents detailing the charges and evidence against them.
They also say they have been subjected to bright lights, hoods and insults from prison guards while in custody.
"It appears the UAE authorities will drag scores of citizens through a shamelessly unfair judicial process that makes a mockery of justice," the international campaign group Human Rights Watch said in a statement.
It has urged US Secretary of State John Kerry to use his visit to the UAE this week to raise concerns over a fair trial.
The 94 are being tried in the UAE's highest court, which means the decision is final with no right of appeal.
The mass trial is due to begin in the United Arab Emirates (UAE) of 94 Islamist activists accused of plotting to seize power in the Gulf state.