SHIFANG, China — Local leaders were all smiles this summer at a groundbreaking ceremony for a vast copper smelting project that seemed like the answer to the chronic unemployment that has plagued this city in northern Sichuan ever since a devastating earthquake in 2008.

But within days, the tree-lined plaza at the heart of the city was packed with thousands of youths, protesting that the $1.6 billion factory would pose a pollution hazard. After two nights of street battles pitting youths against the riot police, city leaders canceled the smelter.

“The environment is more important” than new investments or jobs, said a young woman sitting, on a recent afternoon, at the cafe across the street from the plaza, now empty except for a clutch of retirees gathered under the clock tower.

China’s economic boom over the last three decades has depended overwhelmingly on a build-at-all-costs investment strategy in which pollution concerns, the preservation of neighborhoods and other such questions have been swept aside. But that approach is starting to backfire, posing one of the biggest challenges for the new generation of Chinese policy makers who will take over at the Communist Party Congress, which starts on Thursday.