The Greek port city of Piraeus received 465 migrants on Saturday, the coastguard said, as the clock ticks down on the pending implementation of an EU-brokered deal to send back asylum seekers arriving by boats from Turkey.
The migrants had reached the Greek islands via the Aegean Sea. Another 30 people were expected to be ferried from these islands to mainland Greece later in the afternoon.
Time is running out for so-called irregular migrants in Turkey planning to reach the European Union by the main sea route, which a deal struck on Friday between the bloc and Ankara seeks to close.
Turkey and the EU declared that those arriving on Greek shores can expect to be returned to Turkey as of Sunday. The controversial deal hinges on a one-for-one swap of Syrian migrants, with the EU promising to resettle the same number of Syrians living in Turkey, up to a limit of 72,000.
Meanwhile, 4,000 people are already being accommodated in gym halls and camps in Piraeus, where the mood has been tense as authorities struggle to cope with the influx. Journalists at the scene reported clashes between groups of different ethnic backgrounds.
Most of the migrants in Greece do not want to stay there, but their goal of reaching wealthier countries further north has been thwarted by border closures along the Western Balkan route.
On Friday evening, news of the plans to send back migrants reached those stranded at Idomeni - a squalid, overcrowded camp at the sealed Macedonian border.
One Greek television reporter there spoke of disappointment as well as confusion over what would happen to those who had arrived from Turkey before the deal was struck.
Greek authorities now face another challenge, with broadcasters describing the task of sending people back as "titanic."
Athens daily Kathimerini reported that 4,000 officials would be required to swiftly process the large number of asylum bids in order to determine who should be returned.