Refugees and migrants arriving in Europe will be sent back across the Aegean sea under the terms of a deal between the EU and Turkey that came into force at midnight on Saturday.
The deal has been criticised by aid agencies as inhumane.
On Saturday, European council president, Donald Tusk, resolved sticking points with Turkey's prime minister, Ahmet Davutoglu, before all of the EU's 28 leaders approved the deal at talks in Brussels.
As a result of the deal any migrants arriving after Saturday midnight can expect to be returned to Turkey in the coming weeks.
The UN’s refugee agency said big questions remained about how the deal would work in practice and called for urgent improvements to Greece’s system for assessing refugees.
But thousands of refugees who have already made it to Greece will be resettled in Europe, although they cannot choose where.
German chancellor Angela Merkel urged refugees at Idomeni to move to other accommodation being offered by the Greek authorities.
Some 14,000 people are waiting at the border village in the hope of travelling north. “I want to take the opportunity to tell the refugees at Idomeni that they should trust the Greek government and move to other accommodation where the conditions will be significantly better,” Ms Merkel said.
She added that “from there, Greece will put asylum procedures in motion or redistribution to other European countries will take place”.
In exchange for taking in refugees, Turkey can expect "re-energised" talks on its EU membership, with the promise of negotiations on one policy area to be opened before July. Although this is a climbdown by Turkey, after Cyprus blocked a more ambitious restart of accession talks, Mr Davutoglu said it was "a historic day" for EU-Turkey relations.
But the head of the UN high commissioner for human rights in Europe raised concerns that safeguards intended to protect vulnerable asylum seekers would not be in place in time.
Vincent Cochetel, director of the UNHCR for Europe, said the agreement was legal on paper, but questions remained on how it was implemented.
“For us the proof is in the pudding. Clearly the deal on paper is consistent with international law and standards. The worry is that the safeguards will not be in place on March 20th.”
People claiming asylum needed access to interpreters and the right of appeal, he said, vital elements of a functioning asylum system that Greece has struggled to put in place until now. Implementation “is a big question mark, it is a big challenge”.
Aid agencies accused the EU of failing to respect the spirit of EU and international laws. "This is a dark day for the refugee convention, a dark day for Europe and a dark day for humanity," said Kate Allen of Amnesty International.
Action Aid's Mike Noyes claimed the deal would "effectively turn the Greek islands ... into prison camps where terrified people are held against their will before being deported back to Turkey".
Facing criticism that Turkey is not a safe third country, Mr Davuto?lu defended his country’s record in sheltering 2.8 million refugees.
Those who criticise Turkey “have to provide us with a better option”, he said. The deal was struck as Turkish authorities intercepted hundreds of refugees on land and at sea trying to reach the Greek island of Lesbos in a major operation involving the coastguard and helicopters.
Two Turkish officials in Ankara said the total number detained was about 3,000. The controversial one-for-one deal remains intact: for every Syrian refugee that the EU sends back across the Aegean, a Syrian in Turkey will be given a new home in Europe.
But a cap of 72,000 places has been put on Syrians who will be given asylum in Europe, far short of the 108,000 a year recommended by international aid agencies, if the EU is to do its fair share.
The scheme will be stopped once more than 72,000 people have been settled in Europe, amid concerns among some countries of an “open-ended commitment”. The deal was agreed despite lacerating criticism from Turkey’s president, who attacked Europe’s “shameful” record on refugees as the summit got underway.
Turkish prime minister Recep Tayyip Erdogan, who was not taking part in the Brussels talks, said Europe should look at its own record on refugees before telling Turkey what to do.
"At a time when Turkey is hosting 3 million, those who are unable to find space for a handful of refugees, who in the middle of Europe keep these innocents in shameful conditions, must first to look at themselves," Mr Erdogan told Turkish television.
Approximately 45,000 people are trapped in Greece, including 14,000 who are living in squalid conditions near the Greek-Macedonian border at Idomeni , as they are barred from travelling onwards to central and northern Europe.
Mr Erdogan, who last year threatened to send millions of refugees to Europe on buses and planes, has been frozen out by the EU hierarchy for almost a decade over concerns about his record on human rights and press freedom. In a speech where he criticised the EU for backing Kurdish rebels,
Mr Erdogan also warned that European cities faced repeated terror attacks that have struck Turkey in recent months, some of which have been claimed by Kurdish fighters and others blamed on Islamic State.
“Despite this clear reality, European countries are paying no attention, as if dancing on a minefield.”
Mr Davutoglu was despatched to Brussels to finalise a refugee pact. He secured a promise from the
EU to speed up the disbursement of €3 billion intended to help Syrian refugees in Turkey, with new projects to be agreed this week.
Turkey has pledged that all returned people will be treated in line with international law, including guarantees that they will not be returned to the countries they have fled from.