A first group of migrants were ferried from the Greek islands to Turkey Monday as part of a controversial European Union plan to curb migration to Europe.

Under heavy security, authorities on the Greek islands of Lesbos and Chios put 202 people on boats bound for Turkey — the first to be sent back as part of the plan, which has drawn strong criticism from human rights advocates.

The first vessel arrived later in the Turkish port of Dikili where migrants were taken to red-and-white tents for registration and health checks. About a dozen people stood at the port holding a banner that read "Welcome refugees. Turkey is your home."

A second vessel was expected soon after. Authorities said most of the people in the first batch are Pakistani nationals.

Turkey and the European Union reached a deal last month which stipulates that migrants who reach Greece illegally from Turkey after March 20 will be returned to Turkey unless they qualify for asylum. For every Syrian turned back, a Syrian refugee is to be resettled from Turkey to the EU.

Monday was the designated start date for transfers and marks a symbolic, successful benchmark in the agreement, which has been plagued by concerns over human rights and the adequacy of preparations taken in Greece and Turkey, the primary players in its implementation. The numbers transferred, however, were smaller than initially forecast.

"All of the migrants returned are from Pakistan except two migrants from Syria who returned voluntarily," Giorgos Kyritsis, a spokesman for a government refugee crisis committee, told state TV.

"There is no timetable for returns. Examining (asylum) applications will take some time."

About 4,000 migrants and refugees are being detained on Greek islands since the agreement came into effect March 20.

Kyritsis said 136 migrants were deported from Lesbos and 66 from the nearby island of Chios, where riot police clashed with local residents hours earlier during a protest against expulsions.

"This is the first day of a very difficult time for refugee rights. Despite the serious legal gaps and lack of adequate protection in Turkey, the EU is forging ahead with a dangerous deal," Giorgos Kosmopoulos, head of Amnesty International in Greece, told the Associated Press from Lesbos.

"Turkey is not a safe third country for refugees. The EU and Greek authorities know this and have no excuse."

The operation was supervised by a lieutenant general of the Greek police and occurred peacefully, as ships departed from Lesbos to the Turkish port of Dikili. The deportations started with migrants who did not apply for asylum or had their applications declared inadmissible.

"Even if this first group is not refugees, what we are seeing here is symbolic kick-off of what might be a very dangerous practice of returns to Turkey," Kosmopoulos said.

The first vessel, the Nazli Jale, docked in Dikili accompanied by the Turkish coast guard as a helicopter flew overhead. A second ship, the Lesvos, was due later in the morning.

A total of 50,000 migrants and refugees are stranded in Greece following EU and Balkan border closures, but only those who arrived after March 20 will be detained for deportation.