The UN refugee agency criticized Italy on Friday over conditions at a migrant center on the southern island of Lampedusa.
About 2,000 people are crammed into facilities built for 850, the office of the UN High Commissioner for Refugees said.
"The result is that hundreds of people are now sleeping outdoors under plastic sheeting and adequate reception standards cannot be maintained," UNHCR spokesman Ron Redmond told reporters in Geneva.
Many of those arriving by boat at Lampedusa are Somalis and Eritreans fleeing conflict or seeking a better life. The island is closer to North Africa than to mainland Europe.