Swathes of the Mediterranean city were obliterated by a torrent of water unleashed by a powerful storm that swept down a usually dry riverbed on Sunday night, bursting dams above the city. Multi-storey buildings collapsed with sleeping families inside.
More than 5300 people are believed to have died after the floods in Derna, an official said.
Officials have put the number of missing at 10,000. The UN aid agency OCHA said the figure was at least 5000.
Usama Al Husadi, a 52-year-old driver, has been searching for his wife and five children since the disaster. "I went by foot searching for them ... I went to all hospitals and schools but no luck," he told Reuters, weeping with his head in his hands.
Husadi, who had been working the night of the storm, dialled his wife's phone number once again. It was switched off. "We lost at least 50 members from my father's family, between missing and dead," he said.
The beach was littered with clothes, toys, furniture, shoes and other possessions swept out of homes by the torrent.
Streets were covered in deep mud and strewn with uprooted trees and hundreds of wrecked cars, many flipped on their sides or their roofs. One car was wedged on the second-floor balcony of a gutted building.
"I survived with my wife but I lost my sister," Mohamed Mohsen Bujmila, a 41-year-old engineer, said.Â
"My sister lives downtown where most of the destruction happened. We found the bodies of her husband and son and buried them."
He also found the bodies of two strangers in his apartment.
The devastation is clear from high points above Derna, where the densely populated city centre, built along a seasonal riverbed, was now a wide, flat crescent of earth with stretches of muddy water gleaming in the sun. Buildings were swept away.
Hichem Abu Chkiouat, minister of civil aviation in the administration that runs eastern Libya, told Reuters more than 5300 dead had been counted so far, and said the number was likely to increase significantly and might even double.
The "sea is constantly dumping dozens of bodies", he said by phone.
Tariq Kharaz, a spokesperson for the eastern authorities, said 3200 bodies had been recovered, and 1,100 of them had yet to be identified.
Derna Mayor Abdulmenam al-Ghaithi told Saudi-owned Al Arabiya television the estimated the number of deaths in the city could reach 18,000 to 20,000 based on the number of districts destroyed by the flood.
Rescue teams have arrived from Egypt, Tunisia, the United Arab Emirates, Turkey, and Qatar, al-Ghaithi said.
"We actually need teams specialised in recovering bodies," he said. "I fear that the city will be infected with an epidemic due to the large number of bodies under the rubble and in the water."
Turkey is also sending a ship carrying equipment to set up two field hospitals and 148 medical staff to help with the rescue efforts.
The UN migration agency, the International Organisation for Migration, said at least 30,000 people had been displaced in Derna.
"We need bags for the bodies," Lutfi al-Misrati, the director of the search team, told Al Jazeera.
Rescue operations are complicated by deep political fractures in the country of seven million people that has lacked a strong central government and been at war on-and-off since a NATO-backed uprising that toppled Muammar Gaddafi in 2011.
An internationally recognised Government of National Unity (GNU) is based in Tripoli, in the west, while a parallel administration operates in the east, including Derna.
Libya's Tripoli-based Prime Minister Abdulhamid al-Dbeibah called the floods an unprecedented catastrophe. Libya's Presidential Council head Mohammed al-Menfi has called for national unity.
