LESBOS, Greece (Reuters) - The trial of 24 aid workers, including a Syrian refugee, involved in refugee rescues began on the Greek island of Lesbos on Thursday amid calls from rights groups that authorities drop the charges of spying and disclosing state secrets.

       The aid workers, some of them foreigners, were affiliated with the Emergency Response Center International (ERCI), a nonprofit search-and-rescue group operating on Lesbos from 2016 to 2018. They face up to eight years in prison, convertible into a fine.

       They also face more serious felony charges, still being investigated, of people smuggling, belonging to a criminal group and money laundering, which carry 25-year prison sentences.

       The defendants deny the charges against them and are expected to plead not guilty.

       The European Parliament called it "the largest case of criminalization of solidarity in Europe" in a report in June.

       Among those tried is Sarah Mardini, a Syrian refugee who took an overcrowded dinghy to Greece with her sister Yusra in 2015, at the height of Europe's refugee crisis, and saved the other 19 passengers by pulling their sinking boat to shore for four hours.

       Rights groups such as Amnesty International and Human Rights Watch say the trial is intended to intimidate other aid workers from carrying out their work and have called the charges "farcical" and "politically motivated."

       Mardini and another volunteer, Sean Binder, a German national trained in rescue diving, were arrested in 2018 and spent 107 days in Athens's maximum security prison in pre-trial detention.

       Both left Greece after they were released, and Binder has returned despite his misgivings about the trial. Mardini, who now lives in Germany, is barred from entering the country as a third country national and will be represented by a lawyer.

       (Additional reporting and writing by Karolina Tagaris in Athens; Editing by Angus MacSwan)