Under a pact criticised by refugee agencies and human rights campaigners, Ankara will take back all migrants and refugees who cross the Aegean Sea to enter Greece illegally, including Syrians.
In return, the European Union will take in thousands of Syrian refugees directly from Turkey and reward it with more money, early visa-free travel and progress in its EU membership negotiations.
Two Turkish-flagged passenger boats carrying 136 mostly Pakistani migrants arrived from the island of Lesbos in the Turkish town of Dikili, accompanied by two Turkish coast guard vessels with a police helicopter buzzing overhead.
A third ship carrying 66 people, mainly Afghans, arrived there later from the island of Chios.
The EU-Turkey deal aims to discourage migrants from perilous crossings, often in small boats and dinghies, and to break the business model of human smugglers who have fuelled Europe's biggest influx since World War Two.
EU authorities said none of those deported on Monday had requested asylum in Greece and all had left voluntarily. They included two Syrians who had asked to return to Turkey.
"We didn't see this morning unrest or riots. The operation was organised properly with the sufficient Frontex presence and with enough, very well organised security guarantees," European Commission spokesman Margaritas Schinas told a news briefing in Brussels. He was referring to the EU border management agency Frontex, which has been reinforced by national police and migration experts.
Schinas said the first returns were legal, although Turkey has not yet made changes to its regulations that the EU said were necessary at the time of the deal.
The EU said at the time of the deal that Ankara would need to change asylum laws to give international protection to Syrians who enter from countries other than Syria, and to non-Syrian asylum seekers returned from Greece.
Migrants keep coming
A few hours after the first boat of returnees set sail from Lesbos, Greek coast guard vessels rescued at least two dinghies carrying more than 50 migrants and refugees, including children and a woman in a wheelchair, trying to reach the island.
Altogether, more people arrived on the Greek islands in the 24 hours to Monday morning than were transported to Turkey, Greek authorities said, putting total arrivals at 339, including 173 on Lesbos and 73 on Chios.
"We are just going to try our chance. It is for our destiny. We are dead anyway," said Firaz, 31, a Syrian Kurd from the province of Hasakah who was travelling with his cousin.
Asked if he was aware that the Greeks were sending people back, he said: "I heard maybe Iranians, Afghans. I didn't hear they were sending back Syrians to Turkey... At least I did what I could. I'm alive. That's it."
Two groups of mainly Pakistani men, totalling around 100 people, were also intercepted by the Turkish coast guard on Monday near Dikili, a coast guard official said.
Under the pact, the EU will resettle thousands of legal Syrian refugees directly from Turkey - one for each Syrian returned from the Greek islands.
German police said the first 32 Syrian refugees arrived in Hanover on two flights from Istanbul on Monday under the deal. The European Commission said more resettlement flights were due to Finland on Monday and the Netherlands on Tuesday.
German Interior Minister Thomas de Maiziere said on Sunday that the "high point of the migrant crisis is behind us", but many migration experts say the pressure to reach Europe will continue, possibly via other routes.
A few dozen police and immigration officials waited outside a small white tent on the quayside at Dikili as the returned migrants disembarked one by one, before being photographed and having their fingerprints taken behind security screening.
The returnees from Lesbos were mostly from Pakistan and some from Bangladesh and had not applied for asylum, said Ewa Moncure, a spokeswoman for EU border agency Frontex.
Turkish EU Affairs Minister Volkan Bozkir said any Syrians returned from Greece would be sent to the southern city of Osmaniye, around 40 km (25 miles) from the Syrian border.
For non-Syrians, Turkey would apply to their home countries and send them back systematically, Bozkir said in an interview with Turkish broadcaster Haberturk.
Rights groups and some European politicians have challenged the legality of the deal, questioning whether Turkey has sufficient safeguards in place to defend refugees' rights and whether it can be considered safe for them.
Amnesty International's deputy director for Europe, Gauri van Gulik, visiting Lesbos to monitor the returns procedure, told Reuters: "It's almost based on the assumption Turkey is a safe country for refugees, and we've documented very clearly that it is not right now."
Amnesty last week accused the Turkish army of having turned back thousands of Syrians trying to flee their country in the last few months, sometimes using force.
"The most important thing we lose sight of is that these are individuals who are fleeing horrific scenes of war and we're playing some kind of ping pong with them," van Gulik said.
EU spokesman Schinas said Ankara had provided "assurances" and an amendment to its temporary protection regulation was in the works for Syrians returning from abroad. EU Migration Commissioner Dimitris Avramopoulos was in Turkey on Monday to discuss outstanding issues, including rights of non-Syrians.
Turkey insists it is meeting its international obligations. The EU was determined to get the programme under way on schedule despite such doubts because of strong political pressure in northern Europe to deter migrants from attempting the journey.
There were small protests as the returns got underway.
On Lesbos, a small group of protesters chanted "Shame on you!" when the migrant boats set sail as the sun rose over the Aegean. Volunteer rescuers aboard a nearby boat hoisted a banner that read: "Ferries for safe passage, not for deportation."
The governor of Turkey's Izmir province, Mustafa Toprak, told reporters that the returned migrants would be taken to Kirklareli near the Bulgarian border, well away from the coast.
"We will not build camps on the Aegean," he said adding that those who wanted to stay in Turkey could apply to do so.
Each migrant was accompanied on Lesbos by a plainclothes Frontex officer. They had been transported in a nighttime operation from the island's holding centre to the port. Greek riot police squads also boarded the boats.
At the Moria holding centre on Lesbos, where more than 2,600 are being held, a group of men gathered behind the barbed wire fence and shouted to journalists, who are barred from the camp.
One, who said he was from Iran, shouted: "Women just cry. All our children and women are sick (with the) flu epidemic."
The United Nations High Commissioner for Refugees (UNHCR) and rights groups have said the deal between the European Union and Turkey lacks legal safeguards.
More than 3,300 migrants and refugees are on Lesbos. About 2,600 people are held at the Moria centre, a sprawling complex of prefabricated containers, 600 more than its stated capacity. Of those, 2,000 have made asylum claims, UNHCR said.