BEIJING — More than international prestige or even economic might, the top priority of China’s leadership is to maintain stability among this nation’s vast and varied population. President Hu Jintao explicitly reaffirmed that goal just last month, telling a Communist Party celebration that “without stability, nothing can be accomplished.”

In the aftermath of a large protest on Sunday in a major metropolis in northeast China, Dalian, that craving for rigid orderliness appears increasingly ephemeral. In the face of ever more sophisticated efforts to control and guide expression, significant protests — and visceral public shows of unhappiness with government — appear to be becoming regular features of life.

By official estimates, 12,000 demonstrators marched in Dalian — by other estimates, many more — to demand the removal of the expensive new Fujia chemical factory, whose Pacific coast sea wall had been breached a week earlier in a typhoon. The plant makes paraxylene, a toxic chemical used to make polyester products. It can cause illness and, if concentrated, death.

The mostly peaceful protest was one of the largest reported in nearly three years. It included the extraordinary scene of the city’s Communist Party secretary standing atop a car, pleading with demonstrators to go home and promising to close and move the $1.5 billion plant. Some responded by demanding a date.