Two dozen humanitarian activists who helped migrants reach Greece three years ago face charges including espionage and criminal membership in a keenly watched trial opening Thursday on the island of Lesbos.

Human Rights Watch this week said the activists had provided "life-saving aid to migrants and asylum seekers" and accused Greek authorities of "criminalising rescuers".

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.