MYTILENE, Greece — They had braved risks and hardships to get to Greece, having crossed the narrow strait from Turkey in flimsy rafts like nearly a million others last year — with hundreds dying along the way.

But on Monday, Greek and European Union officials sent them back — 202 migrants — beginning a central part of a deal worked out with Turkey last month to stem the flow of people making the perilous journey to European shores.

In this port on the island of Lesbos, as the sun rose over the Aegean Sea, more than 100 officers from the European border agency, Frontex, marched 136 migrants onto two ferries bound for the Turkish town of Dikili. Once there, the migrants were taken into tents for processing and then loaded onto buses — to where, Turkish officials would not say.

An additional 66 migrants were deported from the island of Chios, where riots broke out last week among asylum seekers fearing deportation. In all, Greek officials said those deported were mostly Pakistanis and Afghans, though they also included two Syrians, who had not asked for asylum, the officials said.