Police on the Greek island of Lesbos have begun placing migrants and refugees on boats bound to Turkey, the first to be deported under a European Union plan to limit the amount of migration to Europe.

Under heavy security, the first 135 migrants were being escorted onto the boats as dawn broke Monday by officers from the EU border protection agency, Frontex, to nearby ports on the Turkish coast, under the plan which has been strongly criticized by human rights groups.

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

On the nearby island of Chios, riot police clashed with local residents hours earlier during a protest against deportations planned there.