The police officer in charge of bringing five Emirati activists to the hearing told the UAE State Security Court that they "refused to come," without offering any further explanation, an AFP reporter said.
In a letter posted online on Saturday, one of the accused, economics lecturer Nasser bin Gaith, said he had decided to boycott the trial after having agreed with his co-accused that they would not receive "a fair trial."
Sunday's hearing was also open to representatives of human rights groups, who had called for the immediate release of the five, saying their trial was "fundamentally unfair."
All four previous sessions since the trial began on June 14 had been held behind closed doors.
Blogger Ahmed Mansoor, Gaith who lectures at the Sorbonne University in the UAE capital Abu Dhabi, and activists Fahid Salim Dalk, Hassan Ali Khamis and Ahmed Abdul Khaleq were all detained in April.
The UAE attorney general said at the time they were being held on suspicion of "committing crimes of instigation, breaking laws and perpetrating acts that pose a threat to state security, undermining the public order, opposing the government system, and insulting the president, the vice president and the crown prince of Abu Dhabi."
Four international watchdogs, in a joint statement issued ahead of Sunday's hearing, called on the authorities to "drop all charges and free the activists."
"The trial of five activists detained nearly six months ago for 'publicly insulting' the United Arab Emirates president and other top officials is fundamentally unfair," the statement said.
Amnesty International, the Arabic Network for Human Rights Information (ANHRI), Front Line Defenders and Human Rights Watch said "the activists’ trial has been marked by procedural flaws and has violated the most basic defence rights of the accused."
Some of the activists had signed a petition in March calling for political reforms, including direct elections and broadening the powers of the toothless UAE legislature, the Federal National Council.
All five pleaded not guilty at the start of the trial.
The rights groups said the activists, their families and their lawyers had also "received numerous death threats as a result of an ongoing campaign of intimidation by some Emiratis in support of the ruling elite."
"To date, authorities have not investigated the threats or prosecuted those responsible," they said.
"Because the case is being prosecuted under state security procedures, the Federal Supreme Court is hearing the charges in the first instance, affording no right of appeal," they added.
"The court has not allowed the defendants to review the evidence and charges against them, including evidence collected by the state security prosecution during the investigative period."
The UAE, a federation of seven Gulf emirates led by oil-rich Abu Dhabi, has not seen any popular protests calling for reform like those that have swept other Arab countries, including Gulf states Bahrain and Oman.