Greece’s high-security Korydallos prison acknowledges that Sara Mardini is one of its rarer inmates. For a week, the Syrian refugee, a hero among human rights defenders, has been detained in its women’s wing on charges so serious they have elicited baffled dismay.

The 23-year-old, who saved 18 refugees in 2015 by swimming their waterlogged dingy to the shores of Lesbos with her Olympian sister, is accused of people smuggling, espionage and membership of a criminal organisation – crimes allegedly committed since returning to work with an NGO on the island. Under Greek law, Mardini can be held in custody pending trial for up to 18 months.

“She is in a state of disbelief,” said her lawyer, Haris Petsalnikos, who has petitioned for her release. “The accusations are more about criminalising humanitarian action. Sara wasn’t even here when these alleged crimes took place but as charges they are serious, perhaps the most serious any aid worker has ever faced.”

Mardini’s arrival to Europe might have gone unnoticed had it not been for the extraordinary courage she and younger sister, Yusra, exhibited guiding their boat to safety after the engine failed during the treacherous crossing from Turkey. Both were elite swimmers, with Yusra going on to compete in the 2016 Rio Olympics.

The sisters, whose story is the basis of a forthcoming film by the British director Stephen Daldry, were credited with saving the lives of their fellow passengers. In Germany, their adopted homeland, the pair has since been accorded star status.

Facebook Twitter Pinterest Sara and Yusra Mardini with trophies at the 2016 Bambi awards, the main German media awards. Photograph: Axel Schmidt/AFP/Getty Images

It was because of her inspiring story that Mardini was approached by Emergency Response Centre International, ERCI, on Lesbos. “After risking her own life to save 18 people … not only has she come back to ground zero, but she is here to ensure that no more lives get lost on this perilous journey,” it said after Mardini agreed to join its ranks in 2016.

After her first stint with ERCI, she again returned to Lesbos last December to volunteer with the aid group. And until 21 August there was nothing to suggest her second spell had not gone well. But as Mardini waited at Mytilini airport to head back to Germany, and a scholarship at Bard College in Berlin, she was arrested. Soon after that, police also arrested ERCI’s field director, Nassos Karakitsos, a former Greek naval force officer, and Sean Binder, a German volunteer who lives in Ireland. All three have protested their innocence.

The arrests come as signs of a global clampdown on solidarity networks mount. From Russia to Spain, European human rights workers have been targeted in what campaigners call an increasingly sinister attempt to silence civil society in the name of security.

“There is the concern that this is another example of civil society being closed down by the state,” said Jonathan Cooper, an international human rights lawyer in London. “What we are really seeing is Greek authorities using Sara to send a very worrying message that if you volunteer for refugee work you do so at your peril.”

Facebook Twitter Pinterest Protesters in Athens demand the release of Sara Mardini and Sean Binder. Photograph: Orestis Panagiotou/EPA

But amid concerns about heavy-handed tactics humanitarians face, Greek police say there are others who see a murky side to the story, one ofpeople trafficking and young volunteers being duped into participating in a criminal network unwittingly. In that scenario,the Mardini sisters would make prime targets.

Greek authorities spent six months investigating the affair. Agents were flown into Lesbos from Athens and Thessaloniki. In an unusually long and detailed statement, last week, Mytilini police said that while posing as a non-profit organisation, ERCI had acted with the sole purpose of profiteering by bringing people illegally into Greece via the north-eastern Aegean islands.

Members had intercepted Greek and European coastguard radio transmissions to gain advance notification of the location of smugglers’ boats, police said, and that 30, mostly foreign nationals, were lined up to be questioned in connection with the alleged activities. Other “similar organisations” had also collaborated in what was described as “an informal plan to confront emergency situations”, they added.

Suspicions were first raised, police said, when Mardini and Binder were stopped in February driving a former military 4X4 with false number plates. ERCI remained unnamed until the release of the charge sheets for the pair and that of Karakitsos.

Facebook Twitter Pinterest Refugees and migrants wave as they approach Lesbos in March 2016. Photograph: Alkis Konstantinidis/Reuters

Lesbos has long been on the frontline of the refugee crisis, attracting idealists and charity workers. Until a dramatic decline in migration numbers via the eastern Mediterranean in March 2016, when a landmark deal was signed between the EU and Turkey, the island was the main entry point to Europe.

An estimated 114 NGOs and 7,356 volunteers are based on Lesbos, according to Greek authorities. Local officials talk of “an industry”, and with more than 10,000 refugees there and the mood at boiling point, accusations of NGOs acting as a “pull factor” are rife.

“Sara’s motive for going back this year was purely humanitarian,” said Oceanne Fry, a fellow student who in June worked alongside her at a day clinic in the refugee reception centre.

“At no point was there any indication of illegal activity by the group … but I can attest to the fact that, other than our intake meeting, none of the volunteers ever met, or interacted, with its leadership.”

Facebook Twitter Pinterest Life vests are left on the shore by refugees arriving in Lesbos in 2015. Photograph: Angelos Tzortzinis/AFP/Getty Images

The mayor of Lesbos, Spyros Galinos, said he has seen “good and bad” in the humanitarian movement since the start of the refugee crisis.

“Everything is possible,. There is no doubt that some NGOs have exploited the situation. The police announcement was uncommonly harsh. For a long time I have been saying that we just don’t need all these NGOs. When the crisis erupted, yes, the state was woefully unprepared but now that isn’t the case.”

Attempts to contact ERCI were unsuccessful. Neither a telephone number nor an office address – in a scruffy downtown building listed by the aid group on social media – appeared to have any relation to it.

In a statement released more than a week after Mardini’s arrest, ERCI denied the allegations, saying it had fallen victim to “unfounded claims, accusations and charges”. But it failed to make any mention of Mardini.

“It makes no sense at all,” said Amed Khan, a New York financier turned philanthropist who has donated boats for ERCI’s search and rescue operations. To accuse any of them of human trafficking is crazy.

“In today’s fortress Europe you have to wonder whether Brussels isn’t behind it, whether this isn’t a concerted effort to put a chill on civil society volunteers who are just trying to help. After all, we’re talking about grassroots organisations with global values that stepped up into the space left by authorities failing to do their bit.”