Greece is poised to create “prison” island camps, say aid groups amid growing criticism of government plans to overhaul refugee reception centres on Aegean outposts facing Turkey.

As the UN refugee agency’s top official, Filippo Grandi, prepared this week to fly to Lesbos, where almost 16,000 people are crammed into a single facility, Athens was criticised for adopting legislation in contravention of basic human rights.

Disquiet mounted as the centre-right administration, which was elected on a tough law and order platform in July, declared that the country again at the forefront of the migration crisis had “reached its limits”.

Announcing measures to tackle a significant increase in arrivals, not seen at such levels since 2015 when nearly a million Syrians entered Europe via the isles, it promised future policies would be defined by deterrence.

Under the scheme, closed installations will replace vastly overcrowded, open-air camps; land and sea borders will be reinforced with about 1,200 more guards and extra patrol vessels and deportations stepped up.

“We are in the eye of the storm,” said the prime minister, Kyriakos Mitsotakis, conceding that pressure on Greece to patrol its eastern frontiers had risen dramatically in the wake of Europe’s decision to seal off the nation’s northern borders against migrant flows. “The country needs a national strategy.”

With the new structures, which will be built to hold no more than 5,000 people, the era “of shameful scenes” spawned by the deplorable conditions of notorious island camps would, he vowed, finally be replaced “by images of modern, properly functioning installations”.

But international aid groups have overwhelmingly condemned the measures. After criticising asylum legislation also passed this month, they predicted the remodelled facilities would only exacerbate the humanitarian disaster unfolding on Europe’s frontiers.

Martha Roussou, senior advocacy officer for the International Rescue Committee in Greece, said: “The government’s announcements represent a blatant disregard for human rights. The creation of closed facilities will simply mean that extremely vulnerable people, including children, will be kept in prison-like conditions, without having committed any crime.”

The Greek branch of Amnesty International called the plans “outrageous”. Likening Lesbos’s infamous Moria refugee camp to a “human rights black hole”, it said: “In reality, we are talking about the creation of contemporary jails with inhumane consequences for asylum seekers, and more widely, negative consequences for the Aegean islands and their inhabitants.”

About 37,000 asylum seekers are trapped on islands that since the summer have been targeted with renewed vigour by traffickers.

With Greece being lashed by rainstorms as winter intensifies, groups have increasingly raised the alarm over what many are calling a humanitarian disaster. Officially, reception facilities on Samos, Lesbos, Chios, Kos and Leros have a capacity to accommodate about 5,400 people.

The number of men, women and children making the treacherous sea crossing from Turkey has risen by 73% this year, according to the UN refugee agency, UNHCR. It said the vast majority are refugees fleeing persecution and war.

The influx marks the biggest jump in arrivals since March 2016 when the EU struck a landmark accord with Ankara to curb flows. The deal, which until this year resulted in the number of arrivals dropping significantly, forced asylum seekers to remain on islands until their claims were cleared. Overwhelmed services are dealing with a backlog of 70,000 asylum requests nationwide.

After visiting the camps last week, Médecins Sans Frontières’ international president, Christos Christou, said: “I’ve been truly shocked and devastated by the extent of the emergency. Men, women and children are trapped in endless drama … In Moria on Lesbos there’s one latrine per 200 people. In Samos, one latrine per 300. This human tragedy needs to end now and it can if Greece and Europe choose to enact a responsible migration system and end these containment policies.”

With local island communities equally under pressure, Mitsotakis has pledged that by early next year 20,000 people will be transferred to newly built camps on the mainland. Asylum processing is also to be accelerated, with an extra 500 staff hired to vet procedures.

On Sunday, Mitsotakis addressed the issue of unaccompanied minors, thought to number more than 4,000. He said special steps would be taken to “close the wound” of youngsters also in Greece.

But while those measures are considered long overdue, the government’s plan to replace camps with closed facilities, that will detain asylum seekers alongside those whose claims have been rejected, has faced particular criticism.

The UNHCR’s representative in Greece, Philippe Leclerc, told the Guardian he was particularly concerned by the message of deterrence the new policies conveyed.

He said: “We need the islands to be decongested for asylum procedures to be processed and so welcome such action. But we don’t see the need for asylum seekers to be kept in closed centres. Most of those coming in are from Afghanistan and Syria. They have the profiles of refugees and are deserving of asylum.”

In a nation that had previously exhibited sympathy and compassion towards refugees, attitudes are hardening. In sharp contrast to its leftist predecessor, the Mitsotakis government is determined to stop would-be migrants in their tracks. Government spokesman Stelios Petsas outlined the measures last week: “A clear message should be sent to those planning, or thinking of coming to the country illegally when they aren’t entitled to asylum. They should realise … if they give money to a trafficker to bring them to Greece they will lose it.”

With the influx showing no sign of abating – about 643 refugees and migrants were rescued in the Aegean over the last 24 hours – arrivals are outpacing transfers, and the government also faces tough resistance from Greeks.

In Chios, Lesbos and on the mainland, the prospect of the new detention centres has been met with anger, with locals staging protests, blockading the arrival of coaches carrying refugees and promising to step up opposition in the months ahead.