The Greek government is bracing itself for violence ahead of the European Union implementing a landmark deal that, from Monday, will see Syrian refugees and migrants being deported back to Turkey en masse.

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Rioting and rebellion by thousands of entrapped refugees across Greece has triggered mounting fears in Athens over the practicality of enforcing an agreement already marred by growing concerns over its legality. Islands have become flashpoints, with as many as 800 people breaking out of a detention centre on Chios on Friday.

Some 750 migrants are set to be sent back between Monday and Wednesday from the island of Lesbos to the Turkish port of Dikili.

Facebook Twitter Pinterest A woman feeds pigeons at the port of Piraeus near Athens where migrants are camped out. Photograph: Yorgos Karahalis/AP

“We are expecting violence. People in despair tend to be violent,” the leftist-led government’s migration spokesman, Giorgos Kyritsis, told the Observer. “The whole philosophy of the deal is to deter human trafficking [into Europe] from the Turkish coast, but it is going to be difficult and we are trying to use a soft approach. These are people have fled war. They are not criminals.”

Barely 24 hours ahead of the pact coming into force, it emerged that Frontex, the EU border agency, had not dispatched the appropriate personnel to oversee the operation. Eight Frontex boats will transport men, women and children, who are detained on Greek islands and have been selected for deportation, back across the Aegean following fast-track asylum hearings. But of the 2,300 officials the EU has promised to send Greece only 200 have so far arrived, Kyritsis admitted.

“We are still waiting for the legal experts and translators they said they would send,” he added. “Even Frontex personnel haven’t got here yet.” Humanitarian aid also earmarked for Greece had similarly been held up, with the result that the bankrupt country was managing the crisis – and continued refugee flows – on very limited funds from the state budget.

Facebook Twitter Pinterest Migrants hold hands as they block the highway near the town of Polykastro in northern Greece in protest at the closure of the border with Macedonia. Photograph: Bulent Kilic/AFP/Getty Images

On Saturday overstretched resources were evident in the chaos on Chios where detainees, fearing imminent deportation, had not only run amok, breaking through razorwire enclosing a holding centre on the island, but in despair had marched on the town’s port. In the stampede three refugees were stabbed as riot police tried to control the crowds with stun guns and teargas. The camp, a former recycling factory, had been ransacked, with cabins and even fingerprint equipment smashed.

“If they make me go back to Turkey I’ll throw myself and my family into the sea,” said Mustafa, a Syrian waiting with his wife and children at the port of Chios told Agence France-Presse. “We went from hell to hell.”

“This is what happens when you have 30 policemen guarding 1,600 refugees determined to get out,” said Benjamin Julian, an Icelandic volunteer speaking from the island. “I witnessed it all and I know that all the time they were chanting ‘freedom, freedom, freedom’ and ‘no Torkia [Turkey], no Torkia’. That is what they want and are determined to get.”

In the mayhem that had ensued, panic-stricken local authorities had been forced to divert the daily ferry connecting the island with the mainland for fear it would be stormed.

Similar outbreaks of violence had also occurred in Piraeus, Athens’ port city, where eight young men had been taken to hospital after riots erupted between rival ethnic groups on Wednesday.

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With tensions on the rise in Lesbos, the Aegean island that has borne the brunt of the flows, and in Idomeni on the Greek-Macedonia frontier where around 11,000 have massed since the border’s closure, NGOs warned of a timebomb in the making. Hopes of numbers decreasing following the announcement of the EU-Turkey deal have been dispelled by a renewed surge in arrivals with the onset of spring.

Official figures showed that 52,147 refugees and migrants were stranded in the country at the weekend, with 6,129 registered on Aegean islands that had been almost completely evacuated after the accord was reached on 20 March.. Last year, more than 1.1 million irregular migrants streamed into Europe with over 850,000 pouring into the continent through Greece.

Pleas from Athens to fellow EU member states to reopen the Balkan route have fallen on deaf ears.