Migrants and refugees protested on a Greek island and rights groups raised legal objections three days before a disputed EU deal to return rejected asylum seekers from Greece to Turkey was due to go into action, with neither side completely ready.

Hundreds of migrants and refugees on the island of Chios tore through a razor wire fence surrounding their holding center and set off for the port in protest against planned deportation, police said. Police did not immediately intervene.

Clashes broke out at the site late on Thursday, during which windows were smashed and 10 people were injured lightly, a police official said. Some 300 women and children broke out of the camp on Friday carrying their belongings.

"They say that they don't want to go back to Turkey and that they are afraid for their safety after yesterday's clashes between migrants in the hot spot," a police official said, using the EU term for registration centers that have become detention camps.

The tension on Chios raised the possibility of resistance when the EU-Turkey plan to send back all migrants and refugees who have reached the Greek islands since March 20 goes into effect from Monday.

Although arrivals have slowed, more than 1,900 people have crossed from Turkey to Greece so far this week and a total of 5,622 have been registered since March 20.

The European Union plans to send hundreds of police and migration officers to Greece over the weekend to help carry out the first returns under a deal meant to end the uncontrolled influx of migrants.

More than a million people fleeing war and poverty in the Middle East and beyond have poured into Europe in the last year, most ending up in Germany, triggering a political backlash and pitting EU governments against each other.

However, the U.N. refugee agency and rights group Amnesty International raised objections, with Amnesty accusing Turkey of sending thousands of people trying to flee Syria back into the war-racked country in recent months.

"In their desperation to seal their borders, EU leaders have wilfully ignored the simplest of facts: Turkey is not a safe country for Syrian refugees and is getting less safe by the day," said John Dalhuisen, Amnesty International's director for Europe and Central Asia.

The European Commission said it was investigating the Amnesty charge and would raise the issue with the Turkish authorities, who had promised to apply the principle of non-refoulement under the Brussels pact.

"SERIOUS GAPS"

UNHCR spokeswoman Melissa Fleming told a Geneva news briefing there were still serious legal gaps in both Greece and Turkey and urged all sides to ensure all safeguard were in place before any returns begin.

The Greek parliament was set to adopt a bill amending the country's asylum laws to enable asylum seekers and other migrants to be sent back to so-called safe countries, without explicitly naming Turkey.

Migration Minister Yiannis Mouzalas sought to reassure lawmakers of the ruling left-wing Syriza party that Athens would not be party to violating migrants' human rights.

"A blame-game against our country is starting, that, based on the new agreement we will encroach on human rights," he told parliament. "I assure you – and I believe this will relieve everyone – that we will strictly adhere to human rights procedures as stipulated by international law and the Geneva Convention."

There was no indication that Turkey was about to change its regulations to grant international protection to non-Syrians returned from the Greek islands as stipulated in the EU deal.

The Turkish parliament was in session on Friday but officials said there were nothing on the agenda relating to the migration agreement. It is not due to sit again until Tuesday.

Any new legislation would need to be signed by President Tayyip Erdogan, who is on an official visit to the United States until at least Sunday, although Prime Minister Ahmet Davutoglu could in theory sign an executive order.

The European Commission continued to put an optimistic face on the implementation plan after EU special envoy Maarten Verwey held talks in Ankara on last-minute preparations.

"Preparations are now well under way to ensure that returns of persons whose asylum claims have been declared inadmissible, and those who have not claimed protection, can start in line with the resettlement of Syrians from Turkey on April 4," Commission spokeswoman Mina Andreeva told a briefing.

Asked about the absence of Turkish guarantees on the treatment of non-Syrians, she said no one who would not benefit from international protection would be returned to Turkey.

Altogether some 53,000 migrants and refugees are trapped in Greece in deteriorating conditions after its northern neighbors closed their border to bar the route northwards to Germany.

The UNHCR said conditions on the islands of Lesbos and Samos and at the Athens port of Piraeus and Idomeni at the border with Macedonia were worsening.

"The risk of panic and injury in these sites and others is real," UNHCR spokeswoman Fleming said.