• They gathered on Monday night at the city's landmark Sahaba Mosque, many chanting for top officials in Libya's eastern government to be sacked.
• Internet and telephone access have also been shut down and journalists ordered to leave in a media crackdown.
The mayor's home in the Libyan city of Derna has been burnt down, as hundreds of protesters demanded answers for last week's catastrophic flood.
They gathered on Monday night at the city's landmark Sahaba Mosque, many chanting for top officials in Libya's eastern government to be sacked.
Derna's whole city council has now been dismissed.
Internet and telephone access have also been shut down and journalists ordered to leave in a media crackdown.
More than 10,000 people are officially missing after two old and dilapidated dams burst, flooding the city.
Figures given for the number of people known to have died have varied widely but the UN says it has confirmed close to 4,000 deaths.
The UN now says one of its teams has been refused permission to enter Derna.
"We can confirm that search and rescue teams, emergency medical teams and UN colleagues who are already in Derna continue to operate," Najwa Mekki, of the UN's humanitarian body OCHA, told Reuters news agency on Tuesday.
"However, a UN team was due to travel from Benghazi to Derna today but were not authorized to proceed," she added.
The home of Derna's mayor, Abdulmenam al-Ghaithi, has become a focal point for people's anger.
Residents say they were not sufficiently warned by officials, who they believe must have known a huge amount of rainfall was coming.
They say they were also given a stay-at-home warning rather than being told to evacuate, although officials deny this.
Since the ousting of long-time leader Muammar Gaddafi, Libya has been riven by power struggles and currently has two governments - a UN-recognised one based in Tripoli, and another in the country's east backed by warlord Gen Khalifa Haftar.
He has been calling the flooding a natural disaster but many Libyans disagree, saying the eastern government had neglected the dams despite prior warnings about their fragile condition.
Speaking from his hospital bed in Benghazi, 48-year-old Abdelqader al-Omrani told the AFP news agency that he and other people living near the dams had "warned the municipality and demanded repairs" after spotting leaks two years ago. "They [now] have our deaths on their conscience," he said.
Scientists from the World Weather Attribution group said Libya's conflict and poor dam maintenance had turned extreme weather into a humanitarian disaster, but noted that up to 50% more rain pounded eastern Libya because of global warming caused by human activity.
On Tuesday, the day after the protests, a minister in eastern Libya's government announced that all journalists had been asked to leave Derna, and accused them of hampering the work of rescue teams.
"Have no doubt, this is not about health or safety, but about punishing Dernawis [Derna's residents] for protesting," said Emadeddin Badi of the Atlantic Council think tank, in a post on X (formerly Twitter).
In addition to a large international aid effort, parts of Libya where, until recently, militias had been fighting each other are now sending volunteers and their own private vehicles with food, water, medicine and bedding.
But humanitarians warn of a brewing public health crisis and demonstrators say they need more aid.
And with their most vital possessions washed away by the water, they also want processing facilities set up to replace lost passports and identity documents.
Monday's rallies at the Sahaba Mosque - itself partially damaged by flooding - were the biggest seen since the floods hit, and there are suggestions the protest has some institutional backing.
"The location of the protest, the Sahaba Mosque, is normally cordoned off as part of the rescue area - so how come all of a sudden all the public was allowed to go [there]?" Claudia Gazzini of International Crisis Group in Libya told BBC Newsday.
"It makes me think that it wasn't necessarily just a spontaneous outburst of anger."