Two of the defendants, Syrian refugee Sarah Mardini and Irish national Sean Binder, have already spent over three months in police custody and face five-year prison sentences.
Two dozen humanitarian activists, who helped refugees reach Greece three years ago, face charges including espionage and criminal membership.
Their trial is set to open on Thursday on the island of Lesvos.
Two of the defendants, Syrian refugee Sarah Mardini and Irish national Sean Binder, have already spent over three months in police custody and face five-year prison sentences over the incident, their lawyer Haris Petsikos told AFP.
But the pair – who were conditionally released in December 2018 and immediately left Greece – are also in line for a related felony investigation which will be tried separately.
Overall, 24 activists are on trial for their alleged affiliation with Emergency Response Centre International (ERCI), a non-profit search-and-rescue group that operated on Lesvos and in Greek waters from 2016 to 2018.
Mardini, who now lives in Berlin, has a seven-year ban on returning to Greece and will not attend Thursday’s trial. Binder has stated he will be present.
Mardini told Human Rights Watch she was “scared” to volunteer again.
“At least we’re out of detention now but we want this to be over. You get so exhausted. This has been a dark three years,” she said.
Mardini had herself travelled by boat from Turkey to Greece in 2015 as an asylum seeker from Syria.
When the engine failed, she and her younger sister Yusra, who swam for the Refugee team at the 2016 and 2020 Summer Olympics and is now a UN goodwill ambassador, helped save others on board by swimming and keeping the boat afloat until it reached Lesbos.
Mardini later enrolled at Bard College Berlin and took leave from her studies for a semester to return to Lesbos as a volunteer with ERCI.
She was arrested in August 2018 just as she was about to fly back home.
Binder was arrested the same day.
The prosecution and investigation have been described in a European Parliament report as “currently the largest case of criminalisation of solidarity in Europe.”
HRW this week said the activists had provided “life-saving aid to migrants and asylum seekers” and accused Greek authorities of “criminalising rescuers”.
The case “seems designed to deter future rescue efforts, which will only put lives at risk”, Bill Van Esveld, associate children’s rights director at Human Rights Watch, said in a statement.
The prosecution has likened ERCI’s search-and-rescue operations to a smuggling crime ring, while its fundraising activities are being investigated as potential money laundering.
Mardini and Binder, as well as other defendants, are charged with espionage based on the police report that their efforts to identify migrant boats in distress included monitoring Greek Coast Guard and Frontex radio channels and vessels.
ERCI was registered as a non-governmental organisation and regularly cooperated with the relevant Greek authorities on rescue missions.
The arrests forced the group to cease its operations, including maritime search and rescue, and providing medical care and non-formal education to migrants and asylum seekers.