Such protests, termed "mass incidents" by the government, occur regularly around China, sparked by traffic accidents to industrial pollution and official abuses of power. Outrage is often exacerbated by perceptions of special treatment for the rich and powerful and by distant and unresponsive autocratic leaders appointed from above by the ruling Communist Party.
The threat of violence has prompted massive fiscal outlays for the police and other internal security measures, spending on which now exceeds the defense budget, the world's second largest after that of the U.S.
Photos from Yuyao posted to various Chinese websites showed protesters smashing vehicles and attacking city offices. Some were bleeding from the head after apparently being clubbed by riot police who were shown massed in their hundreds in front of city hall.
City officials on Wednesday either said they had no information or did not answer phones. The protest was ignored by local Chinese-language media and the Yuyao government's website, which instead was filled with glowing reports on the city's flood response work.
However, the head of the Zhejiang provincial Communist Party Organization Department, Cai Qi, called on his microblog for a rational response to the flooding, and said officials had been working all-out to deal with the disaster.
"Who says the leaders have been useless?" Cai wrote. "The leaders and cadres have been working their hardest to deal with this unprecedented disaster and provocations resulting in radical talk aren't in anyone's interests."
It wasn't clear who, if anyone, had led the protests, news of which had been scrubbed from the Chinese Internet by government censors.
However, Yuyao residents reached by telephone said the situation in the city was calm on Wednesday. They said they were still waiting for government officials to visit them to discuss financial assistance.