More than 250 migrants are believed to have drowned after two partially submerged dinghies were found by a rescue boat.
About 200 people fell off the overloaded vessel that was just 20 nautical miles (37km) in the Mediterranean Sea from Libya when the accident occurred on Wednesday.
The crew of the Phoenix aid boat, chartered by the Malta-based NGO Migrant Offshore Aid Station (MOAS), had begun the rescue and were distributing lifejackets when many of those on deck fell into the water, perhaps knocked off balance by a wave.
“It’s not a scene from a horror movie, it’s a real-life tragedy that is taking place today at the gates of Europe,” said Chris Catrambone, MOAS co-founder, who was aboard the Phoenix.
With the help of an Italian coastguard ship and several commercial ships, rescuers raced to drag as many people as possible from the water, while a helicopter and military aircraft dropped lifeboats.
The incident was one of 15 rescue operations under way on Wednesday off Libya’s Mediterranean coast.
On Tuesday, the coastguard coordinated the relief of about 1,500 people, while their Libyan counterparts intercepted 237 others.
The sea channel between Italy and Libya is the world’s busiest and most dangerous sea-migration route.
The number of refugees and migrants arriving in Italy has surged this year by more than 30 percent in comparison with the same period last year, with 46,000 people arriving so far.
With most departures coming in the warm summer months, the trend points to around 250,000 people arriving over the course of 2017.
But of every 39 people who survive the dangerous journey across the Mediterranean, one dies, according to the International Organisation for Migration (IOM).
More than 5,000 migrants and refugees died crossing the Mediterranean last year, according to the IOM.