Greek authorities are struggling to cope with rising tension on islands where pressure from a new influx of refugees and migrants has reached a critical point.

Friction is growing between local people and asylum seekers landing in boats from Turkey. Last week the region’s most senior official likened the situation on Lesbos to a “powder keg ready to explode”. Kostas Moutzouris, governor of the north Aegean, said: “It’s crucial that a state of emergency is called.”

More than 42,000 men, women and children are estimated to be on Lesbos, Samos, Chios, Leros and Kos. Unable to leave because of a containment policy determined by the EU, they are forced to remain on the islands until their asylum requests are processed by a system both understaffed and overstretched.

Aid groups have repeatedly called for the islands to be evacuated. On Friday an estimated 20,000 refugees were on Lesbos, forced to endure the grim reality of Moria, a camp designed to host 3,000 at most.

“They are living in squalid, medieval-like conditions … with barely any access to basic services, including clean and hot water, electricity, sanitation and healthcare,” said Sophie McCann, Médecins Sans Frontières’ advocacy officer. “On a daily basis our medical teams are treating the consequential deterioration of health and wellbeing.”

But she added that the local people had also been given short shrift. “The Lesbos community has been abandoned by its own government for almost five years to deal with the consequences of a failed reception system. Like the refugee community, it is tired.”

As anti-immigrant sentiment has surged, vigilante groups believed to be infiltrated by supporters of the far-right Golden Dawn party have surfaced. On Friday seven men armed with wooden clubs were arrested in the hilltop village of Moria on suspicion of being members of a gang apparently linked to Golden Dawn.

“People have seen their properties destroyed, their sheep and goats have been slaughtered, their homes broken into,” said Nikos Trakellis, a community leader. “A few years back, when there were 5,000 on the island, things seemed bad enough. Now there’s a sense that the situation has really got out of hand.”

NGOs have also been targeted. In recent weeks cars have been vandalised. Foreigners perceived to be helping refugees have spoken of intimidation. Ciaran Carney, a volunteer filmmaker teaching refugees on the island, said: “There was a week when no one [in the NGOs] wanted to leave their flats. It definitely feels like it could explode and that no one knows what will come next.”