A group of 27 survivors, all men, were plucked to safety on Wednesday, but roughly 100 other passengers who set off with them from Libya were missing and feared drowned, Doctors Without Borders (MSF) said.
Along with two other shipwrecks this week, the latest incident pushed the toll to 18 confirmed dead and 340 missing, in what was already the most lethal year ever recorded for migrant deaths at sea.
The survivors rescued on Wednesday by a British Navy ship, described being stripped of their sole means of survival by the men they had paid for safe passage.
They had set off before dawn on Monday from a beach close to Tripoli. After several hours the traffickers, travelling aboard a separate boat, ordered them at gunpoint to hand over life jackets they had paid for, as well as the boat engine, and left them without a satellite phone to call for help.
"At that point I thought we were going to die", said Abdoullae Diallo, 18, according to MSF. "Without a motor, we couldn't go far. A trafficker told us we would be rescued but I felt like we were going to die."
The overcrowded dinghy began rapidly taking on water and deflated. Tossed for two days and nights on rough seas, some passengers fell overboard, while others succumbed to exhaustion.
By the time the British Royal Navy's HMS Enterprise - engaged in the anti-trafficking Sofia operation - found them, just 27 people were left alive, clinging to what was left of the dinghy.
"They are exhausted, shocked and traumatized," MSF coordinator Michele Delaro told AFP by satellite telephone from the Bourbon Argos rescue ship.
The survivors, who hail from Gambia, Guinea, Mali, Senegal and Sierra Leone, along with eight bodies retrieved from the dinghy were transferred to the MSF ship, which had just returned to the area after disembarking nearly 800 other migrants on the Italian island of Sicily.
The week's succession of tragedies at sea started with the recovery of just 15 of 150 people aboard one rickety dinghy on Monday. On Tuesday 23 were found on another boat that initially had 122 aboard.
The first group of survivors were brought to Catania, in Sicily, while the second group were expected to arrive on Italy's mainland in the port of Reggio Calabria
Search for the missing
Some were children. "One young boy has been weeping, asking for his mother," Mathilde Auvillain, a spokeswoman for SOS Mediterranee told AFP.
"Another has written a list of names of the people travelling with him and re-reads it over and over. He wants to know if his friends are on the boat or in the sea," she said.
Over 3,350 people have been rescued from crowded and unseaworthy dinghies since Saturday, according to the Italian coast guard, including 146 people on Thursday.
The total is the same as for the whole month of November 2015 and, following a record number of arrivals in October, shows departures from Libya are not being deterred by worsening weather in the Mediterranean.
Meanwhile the number of rescue boats patrolling the crossing zones has declined as winter approaches, due to financial and logistical constraints.
Since the start of the year, over 167,000 people have been brought to safety in Italy, a figure that has already passed the 153,000 number recorded in 2015 and is closing in on the 170,000 figure recorded in 2014.
"The unending rescues and high number of victims in recent days show how critical the situation is in the Mediterranean, it is a real humanitarian catastrophe that is taking place before our very eyes", said Sophie Beau, head of SOS Mediterranee.
"Europe urgently needs to take responsibility and put in place an adequate response" to the crisis, she said.
By Fanny Carrier and Ella Ide