Buses carrying hundreds of migrants for deportation to Turkey arrived at the Greek island ports of Lesbos and Chios on Monday morning.
The expulsion launches a controversial European Union (EU) deal to send migrants back across the Aegean Sea that has been criticised by rights groups on ethical grounds.
Two Turkish leisure vessels on Lesbos and another one on Chios are waiting to pick up the migrants, who are to be escorted by police from EU border agency Frontex.
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On Lesbos, where five buses arrived, crews were earlier seen loading supplies onto the ships — a small ferry and a catamaran.
At the port of Chios, a single busload a few dozen activists gathered near the embarkation site chanting 'Freedom', an AFP photographer said.
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Turkish Interior Minister Efkan Ala has said his country is ready to receive 500 migrants on Monday and Greek authorities have provided 400 names, although these numbers could change.
The EU signed the controversial deal with Turkey in March as it wrestles with the continent's worst migration crisis since World War II, with more than a million people arriving last year.
Under the agreement, designed to halt new arrivals along the most popular route through Turkey, all "irregular migrants" arriving since March 20 face being sent back. Each case is meant to be examined individually.
For every Syrian refugee returned, another Syrian refugee will be resettled from Turkey to the EU, with numbers capped at 72,000.
Turkish authorities say the first wave of returned migrants will arrive at the resort of Dikili, just opposite Lesbos. Tents have been put up by the town's harbourside in anticipation of Monday's arrivals, according to media reports.
The operation to resettle Syrians to Europe under the one-for-one arrangement also starts on Monday.
Germany expects to take in a first group of about 35 Syrians from Turkey on Monday, the German interior ministry said. Several dozen others are expected to arrive in France, Finland and Portugal, according to German government sources.
Campaigners have criticised the deal, with Amnesty International saying Turkey is not a safe country for refugees — a charge Ankara rejects.