The Greek parliament has approved a bill that amends its asylum laws to pave the way for the planned return of asylum seekers to Turkey, according to the nation's parliamentary office.
The law passed late Friday with 169 votes in favour and 107 votes against, thereby enabling the implementation of a controversial agreement between the European Union and Turkey to reduce the influx of migrants.
According to the deal, any migrants who reached the Greek islands from Turkey after March 20 and do not apply for asylum - or fail to qualify - will be returned to Turkey, a major transit country for people trying to reach Europe.
The migration deal, which is scheduled to go into effect on Monday, drew renewed criticism Friday from the Amnesty International human rights organization and from the United Nations refugee agency (UNHCR).
"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 Amnesty's John Dalhuisen.
Amnesty charged in a Friday report that Turkey has been sending back hundreds of refugees to Syria "on a near-daily basis" since January.
The EU-Turkey deal "can only be implemented with the hardest of hearts and a blithe disregard for international law," Dalhuisen added.
Pointing to chaotic conditions in Greece, UNHCR said on Friday that the rights of refugees must be recognized in Greece and Turkey before the European Union can start returning people from one country to the other.
The situation in hotspot centres on Greek islands are deteriorating as migrants face overcrowding, insufficient food supplies and lacking shelter, UNHCR spokeswoman Melissa Fleming said.
Despite the criticism, the European Commission said it still plans for the returns to begin on Monday, arguing that the required legal conditions would be in place by then.
This includes a requirement for Turkey to provide protection to all refugees sent back from Greece and to promise not to return refugees to countries where they are at risk.
The commission is examining Amnesty's allegations and is raising the issue with Ankara, a spokeswoman said. The principle of not returning refugees to a country where they are at risk is a "red line" for the EU, she insisted.
Another spokeswoman said the EU's executive was doing everything to improve the situation in Greek reception centres.
"We are aware that there are tensions, that there are capacity problems - this is precisely why we are there in the process of helping the Greek authorities to manage the situation," she said.
A German government spokesman said on Friday the number of refugees being relocated to Turkey from Greece on Monday would be in the hundreds.
Meanwhile, Greek harbour police were Friday moving migrants in Athens' Piraeus port to regulated camps in Greece. Around 5,300 people remain at the port's crowded waiting areas.
Conditions on the mainland at Piraeus and the Greek-Macedonian border were "dismal," Fleming said, warning that "the risk of panic and injury in these sites and others is real in the current circumstances."
Migrants in the Piraeus port are currently sleeping in departure halls normally used by ferry passengers and in more than 1,000 camping tents put up in the docks. There have been reports of tussles between different ethnic groups as tensions rise.
Meanwhile, Greek newspaper Ta Nea reported that at least 500 migrants had cut through the wire fencing at a so-called hotspot built for new arrivals on the island of Chios at around midday Friday.
Clashes had broken out in the same camp late Thursday between Afghans and Syrians.
The facility has been used to hold migrants headed for Europe since the EU-Turkey deal was struck on March 20. Around 1,500 people are awaiting deportation to Turkey there, according to Ta Nea.