The European Union has sealed a controversial deal with Turkey, intended to halt illegal migration flows to Europe in return for financial and political rewards for Ankara.
The accord aims to close the main route by which a million migrants and refugees poured across the Aegean Sea to Greece in the last year before marching north to Germany and Sweden.
But deep doubts remain about whether it is legal or workable, a point acknowledged even by German Chancellor Angela Merkel, a driving force behind the agreement.
"I have no illusions that what we agreed today will be accompanied by further setbacks. There are big legal challenges that we must now overcome," Merkel said after the 28 EU leaders concluded the deal with Turkish Prime Minister Ahmet Davutoglu on Friday.
"But I think we've reached an agreement that has an irreversible momentum," Merkel said, adding it showed that the EU was still capable of taking difficult decisions and managing complex crises.
Under the pact, Ankara would take back all migrants and refugees, including Syrians, who cross to Greece illegally across the sea.
In return, the EU would take in thousands of Syrian refugees directly from Turkey and reward it with more money, early visa-free travel and faster progress in EU membership talks.
Migrants who arrive in Greece from Sunday will be subject to being sent back once they have been registered and their individual asylum claim processed.
The returns are to begin on April 4, as would resettlement of Syrian refugees in Europe.
While many in Brussels hailed the agreement as a game-changer, Amnesty International decried it as a "historic blow to human rights", saying Europe was turning its back on refugees.
"Guarantees to scrupulously respect international law are incompatible with the touted return to Turkey of all irregular migrants," the rights advocacy group said, criticising Ankara's track-record on human rights.
"Turkey is not a safe country for refugees and migrants, and any return process predicated on it being so will be flawed, illegal and immoral."
Turkey's human rights record has drawn mounting criticism amid a crackdown on Kurdish separatists, arrests of critical journalists and the seizure of its best-selling newspaper.
Dutch Prime Minister Mark Rutte sought to reverse the narrative, saying the idea was to discourage illegal and perilous voyages across the Aegean and open legal paths to Europe instead.
"There is nothing humanitarian in letting people, families, children, step on boats, being tempted by cynical smugglers, and risk their lives," he said.
