Clashes have broken out between migrants and riot police at a detention centre on the Greek island of Lesbos, police said.
Migrants at the Moria camp threw stones and sprayed water during a visit by the Greek migration minister and a Dutch minister, officials told the BBC.
Bins were set on fire and there were reports of injuries. Riot police were conducting operations, police said.
Earlier this month the camp was visited by the Pope.
It was turned into a closed detention centre after a controversial EU deal to send economic migrants back to Turkey was announced.
Under the EU-Turkey agreement, migrants who have arrived illegally in Greece since 20 March are to be sent back to Turkey if they do not apply for asylum or if their claim is rejected.
For each Syrian migrant returned to Turkey, the EU is to take in another Syrian who has made a legitimate request.
Earlier, 49 migrants from Afghanistan, Iran, Pakistan and Myanmar became the third group of migrants to be returned to Turkey under the deal.