Italy's coast guard said it co-ordinated the rescue of some 4,400 migrants in a single day, a record-setting number, as smugglers took advantage of ideal sea conditions off Libya to launch a fleet of overcrowded, unseaworthy boats.

It began with blips on a radar screen, 12 miles off the Libyan coast. As the rescuers approached, they found overloaded wooden vessels and rafts that evoked scenes of the slave trade.