Brought to safety in the Italian ports of Taranto and Pozzallo, survivors told the UN's refugee agency (UNHCR) and Save the Children how their boat sank on Thursday morning after a high-seas drama which saw one woman decapitated.
"We'll never know the exact number, we'll never know their identity, but survivors tell that over 500 human beings died," Carlotta Sami, UNHRC spokeswoman, said on Twitter.
Giovanna Di Benedetto, Save the Children's spokesperson in Sicily, told AFP it was impossible to verify the numbers involved but survivors of Thursday's wreck spoke of around 1,100 people setting out from Libya on Wednesday in two fishing boats and a dinghy.
"The first boat, carrying some 500 people, was reportedly towing the second, which was carrying another 500. But the second boat began to sink. Some people tried to swim to the first boat, others held onto the rope linking the vessels," she said.
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The Sudanese was arrested on his arrival in Pozzallo along with three other suspected people traffickers, Italian media reports said.
"We tried everything to stop the water, to bail it out of the boat," a Nigerian girl told cultural mediators, according to La Stampa daily.
"We used our hands, plastic glasses. For two hours we fought against the water but it was useless. It began to flood the boat, and those below deck had no chance. Woman, men, children, many children, were trapped, and drowned," she said.
"I saw my mother and 11-year old sister die," Kidane from Eritrea, 13, told the aid organisations. "There were bodies everywhere".
A bout of good weather as summer arrives has kicked off a fresh stream of boats attempting to make the perilous crossing from Libya to Italy. Italian news agency Ansa said some 70 dinghies and 10 boats had set off over the past week. Over 15 a day.