Repair work was agreed but never finished and Derna paid the price. Experts say infrastructure projects have long been neglected, with officials focusing instead on lining their own pockets.
Derna, which lost entire neighborhoods and thousands of residents in the deluge, has a history as a cultural and intellectual hub as well as a rebellious streak.
Several people have been arrested and some journalists and aid workers have been shut out of the eastern city of Derna after dams burst, killing thousands. Communications have also been disrupted.
Khalifa Hifter, who oversees a military dictatorship that rules the eastern half of the divided, oil-rich nation, is dominating the response to the disaster and could use it to further entrench his power.
The cries of protesters were part of a rising chorus of calls for answers from leaders across a divided Libya, specifically for an international investigation.