Greek authorities have accused 33 members of non-governmental organisations of facilitating the human trafficking of migrants to Lesbos.
The foreign nationals are alleged to have violated immigration laws by assisting the migrants' transfer to the island from Turkey.
"Under the guise of humanitarian action, [they] provided confidential information ... to refugee flows from Turkey via closed groups and internet applications,'' according to a police statement.
Police say this information included details of when and where on the Turkish coast migrant boats were preparing to depart for Greece, the coordinates of the boats and their direction, the number of people on board, and the situation in the migrant camp of Moria on Lesbos.
The accused are also facing charges of espionage, violation of state secrets, and participation in a criminal organisation.
Two other "third-country nationals" have also been arrested. Authorities have not released the names of those involved or the NGOs.
Authorities are investigating at least 32 cases of migrants arriving on the island and say that the suspects also "hampered the operational work" of Greek coastguard vessels.
Police said the ongoing investigation began several months ago and included Greece's intelligence service and anti-terror police.
Thousands of migrants and refugees arrive on the Greek islands from the nearby Turkish coast each year, with Lesbos being one of the main destinations.
Around 12,000 people were left in need of emergency shelter after the island's camp at Moria burned down earlier this month.
Authorities have built a new camp, consisting of large family tents, on a former shooting range on the Lesbos' coast.