ATHENS, Greece (AP) — Government officials in Greece signaled Saturday that implementation of a migration agreement between the European Union and Turkey could only occur gradually, with key details still to be worked out on how migrants newly arriving from Turkey will be processed and returned.

Prime Minister Alexis Tsipras met ministers and senior officials involved in the migration crisis, hours after the deal was reached in Brussels to send back migrants arriving on the Greek islands to Turkey, starting Sunday.

Hours before that implementation was due to start, it remained unclear how migrants would be sent back. Greek officials had previously suggested that the European border protection agency Frontex could escort chartered private vessels back to Turkey.

Yiannis Balafas, the deputy interior minister, said swift screening procedures in the Greek islands would require additional staff promised by the European Union.

"(Migrants) will be returned after they have been swiftly processed. That is why we need the technical assistance," Balafas told private Mega television.

In central Athens, several thousand migrants and protesters staged protest rallies against the EU-Turkey deal. Afghan migrants marched to the EU commission building in the center of the capital, chanting "Open the Borders" and "We are Human."

The rallies ended peacefully. Protests were also held in other parts of Greece, including Thessaloiniki in northern Greece and on the island of Lesbos. And several thousand people gathered in Spain's northeastern port city of Barcelona to protest against what they called Europe's "racist and uncaring" approach to migrants.

Greece is expecting some 2,300 European experts, including migration officers and translators, to help implement the deal.

"Obviously, none of those people have arrived yet," a government official told the AP, asking not be identified pending official announcements. "What we have at the moment is a political decision. This must now be put into practice."

Migrants on Lesbos and other islands in the east Aegean Sea were being taken by ferry to the mainland ports of Piraeus and Kavala where they will be placed in shelters and eligible for an EU-wide relocation program.

"Migrants on the islands will be moved to mainland shelters, including 2,500 people on Lesbos who are being transported to three different shelter locations," the government officials said.

From Sunday onward, migrants who arrive on the islands will be screened and their identities recorded before being sent back to Turkey.

Germany's interior minister said the Balkan route that migrants have been using to reach Central Europe is "finished" now that Turkey has agreed to take back people who arrive in Greece illegally.

In a statement Saturday, Thomas de Maiziere described the deal between the European Union and Turkey as a "turning point in the refugee crisis."

De Maiziere said Germany will send border police and immigration staff to help Greece process any new arrivals.

His ministry said the number of migrants reaching Germany each day is now down to about 100 a day, a sharp decrease from the thousands who were arriving in the country daily last year.

De Maiziere, a member of Chancellor Angela Merkel's center-right Christian Democrats, said "alternative flight routes must be prevented."

At the Greek-Macedonian border Saturday, no new arrivals were reported by relief agencies at a giant makeshift camp near the border village of Idomeni where about 10,000 migrants are stranded.

Migrants took advantage of a break in bad weather to wash clothes and seek information on the EU-Turkey deal — responding with a mixture of relief and disappointment, with Balkan borders to remain closed but with most migrants already on Greek territory made exempt from plans for swift deportation.

Mohamed Tamer spent three weeks camped out at Idomeni, hoping to travel onto Berlin where his sister lives.

On Saturday, he decided it was time to leave.

"I will try and apply with the EU relocation service," he told the AP before boarding a bus for Athens. "The decision made in Brussels is not clear. What will happen to us? No one cares," he said.

"(EU leaders) should have come here and spent one night in Idomeni before making up their minds."

Early Saturday, several migrants tried to sneak into Macedonia under cover of darkness only to be turned back by Macedonian. After they returned to Idomeni, some of the migrants said had been beaten by the Macedonian border guards.

Anmen Boudin, 17, an Algerian migrant, said he was among a group of 20 men who tried to sneak into Macedonia by climbing the border fence west of Idomeni, only to be quickly surrounded by Macedonian security forces. Boudin said he was now using crutches to walk around because he was repeatedly struck in the legs before being bundled with the other migrants into trucks and returned to Greece.

He said another group of 20 migrants also jumped the fence, but were also turned back and several of them were beaten.

In Idomeni, a group of about 50 migrants — including a legless man from Gaza — continued a protest by the railway line, but did not interfere with the traffic flow.

Macedonia and countries to its north — Serbia, Croatia and Slovenia — along the so-called "western Balkan route" have totally halted migrant flows into Central Europe, even to refugees from war-torn Syria and Iraq. The route had already been off-limits to migrants from Afghanistan and countries in southern Asia, north and east Africa.

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Kantouris reported from Idomeni, northern Greece. Demetris Nellas in Athens and Menelaos Hadjicostis in Brussels contributed.

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Follow Gatopoulos at http://www.twitter.com/dgatopoulos and Kantouris at http://www.twitter.com/CostasKantouris

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Online:

EU Commission Q&A on migration deal: http://europa.eu/rapid/press-release_MEMO-16-963_en.htm