The trial of five members of foreign non-governmental organizations accused of attempted human smuggling was under way at a court on Lesvos on Monday.
The five are three Spanish firefighters, who came to Greece at the peak of the refugee crisis to work for the Spanish group Proem-Aid, and two members of Danish non-government organization Team Humanity.
They are alleged to have attempted to rescue migrants without the authorization of Greek coast guard officers in January 2016.
If convicted, they could face heavy prison terms, according to their lawyers.
Human rights groups have rallied to their defense with Amnesty International on Monday calling for the charges to be dropped, claiming that the five should not be prosecuted for trying to save refugees.