Photographer Aris Messinis, who was able to go on board the vessel, said many of the dead had suffocated.
"It was a wooden vessel and there were about 1,000 people on three levels. I counted 22 bodies and there are still others in the hold," he said by telephone.
Messinis was travelling on the Astral, a ship chartered by Spanish NGO ProActiva Open Arms, which rescues migrants at sea.
Photo: Aris Messinis/AFP
Towards 2000 GMT the Astral rescuers moved back to allow the Italian navy to take care of the survivors and retrieve the bodies, the photographer said.
The deaths came after the Italian coastguard - which is coordinating rescue efforts in international waters north of Libya - said at least 1,800 migrants were rescued off the Libyan coast on Tuesday.
It was yet another day of drama at sea after more than 6,000 migrants, most of them Africans in packed rubber dinghies, were rescued off Libya on Monday.
Nine bodies were found in those operations, including a pregnant woman.
'Panic onboard'
Photo: Aris Messinis/AFP
The Italian coastguard said operations had been carried out on Tuesday to bring migrants to safety from some 30 packed boats.
With numerous rescue boats en route to Italy to unload the huge human cargo collected on Monday, Messinis said the Astral was left alone to deal with the overloaded wooden boat for several hours.
It was aided only by lifeboats dropped by a Spanish military plane until the navy arrived at midday.
"There was panic onboard, there were people jumping into the water," Messinis said.
A mother holds her child after the rescue. Photo: Aris Messinis/AFP
The Astral's rescue operations had begun before dawn and lasted until nightfall.
The latest people to be rescued will add to a total of some 132,000 migrants who have landed at Italy's southern ports so far this year, fleeing conflict, poverty or persecution and hoping to start new lives in Europe.
The new arrivals are mostly Africans who plan to try to get to northern Europe but increasingly are obliged to seek asylum or the right to remain in Italy, where reception centres have been strained to bursting point.
Migrants aboard rubber dinghies during a previous rescue operation. File photo: AFP