The human rights group Amnesty International said migrants were being held "arbitrarily in appalling conditions" at camps on Chios and Lesbos, after interviewing dozens of detainees there.
"On the edge of Europe, refugees are trapped with no light at the end of the tunnel. Amnesty's deputy Europe director Gauri van Gulik said.
"A setup that is so flawed, rushed and ill-prepared is ripe for mistakes, trampling the rights and well-being of some of the most vulnerable people."