Some aid has started to arrive, including from Egypt, but rescue efforts have been hampered by the political situation in Libya, with the country split between two rival governments.
"At first we just thought it was heavy rain but at midnight we heard a huge explosion and it was the dam bursting," Raja Sassi, who survived along with his wife and small daughter, told Reuters news agency.
Libyan journalist Noura Eljerbi, who is based in Tunisia told the BBC she only found out that around 35 of her relatives who all lived in the same apartment block in Derna were still alive after reaching out to a local rescue team.
Libya has been in political chaos since long-serving ruler Col Muammar Gaddafi was overthrown and killed in 2011 - leaving the oil-rich nation effectively split with an interim, internationally recognised government operating from the capital, Tripoli, and another one in the east.
Libya's leading Al-Wasat news website has suggested that failures to properly rebuild and maintain infrastructure in Derna after years of conflict is partly to blame for the high death toll.
"The security chaos and Libyan authorities' laxity in carrying out close monitoring of safety measures [of the dams] led to the catastrophe," it quoted economic expert Mohammed Ahmed as saying.
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