The trial of more than 20 aid workers who helped migrants reach Greece between 2016 and 2018 has been adjourned shortly after opening.
The defendants face a raft of charges, including espionage, forgery and the unlawful use of radio frequencies.
Human rights groups have condemned the trial as "farcical" and politically motivated.
The case was suspended after the judge ruled that the court was not competent to hear the case.
It will now be heard by an appeals court, because one of the defendants is a lawyer and the local court on the island of Lesbos doesn't have the jurisdiction to try lawyers.
One of the accused, Irish aid worker Seán Binder, expressed his frustration at the decision in a statement to BBC News.
"Today's decision is further proof of the absurdity of this case. All we have done is assisting people seeking safety at a time of need. After today's decision, our lives are once again left on hold."
Also among the 24 defendants, 17 of whom are foreign nationals, is Syrian refugee Sarah Mardini, the sister of the Olympic swimmer Yusra Mardini. The siblings came to international attention in 2015 after dragging their refugee boat to safety when the engine failed.
Ms Mardini has been barred from entering Greece to defend herself and has been forced to watch the trial from Germany, where she was granted asylum.
Prosecutors allege that the aid workers, who were affiliated with a search and rescue group, monitored the Greek Coast Guard's radio channels and used a vehicle with a fake military licence plate to enter restricted areas on Lesbos.
If found guilty they could face eight-year prison sentences.
Several also remain under investigation for a number of other felonies, including human smuggling, and if convicted could face further sentences of up to 25-years.
Speaking following today's ruling, Giorgos Kosmopolous, Senior Campaigner on Migration for Amnesty International, told BBC News that the charges levelled against the activists were "farcical" and that the adjournment left the defendants' lives in "limbo".
"We call for the Greek authorities to uphold their human rights, drop the charges to let them get on with their lives," he said.
The trial comes as Greece's conservative government, led by Kyriakos Mitsotakis, has clamped down on human rights groups in an attempt to limit the number of migrants entering the country.
Last year it introduced tougher rules for the operation of dozens of international aid groups, which critics say makes it harder to operate in the country.