An estimated 200 people are missing and feared dead; the bodies of 82 people washed ashore on Friday local time.
In one boat, many appeared to have been trapped in the hold when it sank.
The boats left separately on Thursday from the western Libyan town of Zuwara, a major launchpad for smugglers shipping migrants to Italy.
The migrants were from sub-Saharan Africa, Pakistan, Syria, Morocco and Bangladesh, an security official said.