There is growing concern over a lack of preparation for the plan to send migrants back from Greece to Turkey, two days before it goes into effect.
A BBC correspondent who has been to one of the registration sites in Turkey says there are few signs it is ready.
The UN and rights groups have expressed fears about migrant welfare and whether some may be forced back into Syria.
The EU-Turkey deal is aimed at easing the uncontrolled mass movement of people into Europe.
Under the deal, migrants arriving illegally in Greece are expected to be sent back to Turkey from 4 April if they do not apply for asylum or if their claim is rejected.
An EU official said it was hoped that several hundred would be returned on Monday.
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For each Syrian returned, a Syrian migrant in Turkey will be resettled in the EU, with priority given to those who have not tried to enter the EU illegally.
Germany said on Friday it would take the first migrants.
Interior ministry spokesman Tobias Plate said that most would be families with children, with the numbers in the “double-digit range”.
Last year, more than one million migrants and refugees arrived in the EU by boat from Turkey to Greece.
Tens of thousands have been stranded in Greece after northern countries closed their borders.
‘Safe country’ queried
Greek officials told Associated Press that Monday’s returns could start from the island of Lesbos and involve migrants whose asylum claims were considered inadmissible, including those from Pakistan.
Migrants will be taken on buses and put on chartered vessels, under a heavy security escort of one police guard for every migrant, the officials said.
But the BBC’s Mark Lowen, who has been to a proposed registration camp near the western Turkish town of Dikili, says the field earmarked for the centre remains empty.
He says local officials are suggesting those sent back here would only stay temporarily in Dikili before being sent to other areas of Turkey.
Meanwhile, he says, boats are still attempting to cross – 160 migrants were caught by the Turkish coastguard near Dikili on Friday and many more actually reached Lesbos.
Our correspondent says there are grave doubts over the viability of the EU plan, not least due to concerns that migrants sent back could be mistreated, with several aid agencies and EU countries refusing to recognise Turkey as a so-called “safe country of origin”.
A report from Amnesty International has accused Turkey of illegally forcing thousands of refugees back to Syria.
Turkey has denied sending back any refugees against their will.
On Friday the Greek parliament passed by 169 votes to 107 a law allowing migrants arriving in the country to be returned to Turkey.
But there have been violent protests among migrants.
On the Greek island of Chios, hundreds tore down the razor wire fence that was keeping them in and fled the camp.
They walked to the port demanding to continue their journeys to northern Europe.
One migrant said: “Deportation is a big mistake because we have risked a lot to come here especially during our crossing from Turkey to Greece. We were smuggled here from Turkey. We cannot go back.
“We will repeat our trip again and again if need be because we are running away in order to save our lives.”
Tension has also flared at the port of Piraeus near Athens.
The United Nations refugee agency has urged Greece and Turkey to provide more safeguards before the returns begin.
It said conditions were worsening daily for the thousands held on Greek islands.
Key points from EU-Turkey agreement
Returns: All “irregular migrants” crossing from Turkey into Greece from 20 March will be sent back. Each arrival will be individually assessed by the Greek authorities.
One-for-one: For each Syrian returned to Turkey, a Syrian migrant will be resettled in the EU. Priority will be given to those who have not tried to illegally enter the EU and the number is capped at 72,000.
Visa restrictions: Turkish nationals should have access to the Schengen passport-free zone by June. This will not apply to non-Schengen countries like Britain.
Financial aid: The EU is to speed up the allocation of €3bn ($3.3 bn; £2.3 bn) in aid to Turkey to help migrants.
Turkey EU membership: Both sides agreed to “re-energise” Turkey’s bid to join the European bloc, with talks due by July.