ALTERCATIONS between unlicensed street vendors and law-enforcement officers are commonplace in China. Sometimes they escalate into scuffles or riots. But a night-time rampage by hundreds of citizens in the southern city of Kunming, capital of Yunnan province, on March 26th-27th has aroused fresh concerns about a malaise in Chinese cities.

The violence in Kunming reportedly left dozens injured. Ten government vehicles were overturned and some set on fire by crowds enraged by rumours that a vendor had been killed by an officer of Kunming's “City Administration and Law Enforcement Bureau”. This agency, commonly known by its Chinese abbreviation chengguan, is a junior cousin to the police force. It is responsible for matters such as clearing the streets of illegal pedlars and supervising house demolitions. Chengguan officers are renowned for their thuggish, fine-gouging ways.

The vendor, as it turned out, had not been killed. But the rioters could be forgiven for assuming the worst. In the past couple of years even some state-controlled newspapers have made common cause with critics of chengguan activities across the country. In January 2008 a man in the central province of Hubei was beaten to death when he attempted to film officers trying to stop a protest by villagers against a dump for urban waste. “Another citizen has fallen. When will we stand up and restrain the chengguan system?” wrote a newspaper columnist at the time.

The Chinese press has reported others having fallen to the chengguan since: a pedlar left severely brain-damaged after a mauling in Shanghai last July; a man beaten to death in Beijing in October after being accused of illegally using his motorcycle as a taxi. One case prompted a letter to China's legislature. A woman in the province of Sichuan died last November after setting herself on fire in protest when officers burst into her home to enforce a demolition order. In response, a group of Beijing law professors wrote proposing tighter controls on demolition procedures.

Protests triggered by chengguan brutality have rattled the authorities, hypersensitive as they are to any urban unrest that might turn against the government. Last May hundreds of university students protested in the eastern city of Nanjing against the alleged beating of a classmate. The following month police rescued several chengguan who were captured by rioters in a town in the southern province of Guangdong. In Kunming last October protesters put the corpse of a pedicab-driver, who had allegedly been killed by chengguan, on a gurney and wheeled it to a chengguan office where they burned paper as a traditional funeral offering (the authorities said he had died naturally). That same month a Shanghai man became famous when he chopped off part of a finger in protest at what he said was an attempt to frame him as an illegal taxi- driver.

The latest flare-up in Kunming has also attracted considerable press attention. One newspaper website described the eruption as symptomatic of public resentment against local officialdom that could blow up like “a bomb at any time”. Another newspaper attacked the Kunming authorities for releasing only bare details and not taking questions at a press briefing on the incident. A third suggested the official version of events, that the vendor had simply fallen over, might be a “lie” (a word even used in the headline). It quoted witnesses saying an officer had pushed over her pedicab, pinning the woman under it. A gas canister had then rolled on top of her, knocking her unconscious.

In recent weeks, a speech on social unrest by a prominent Chinese scholar, Yu Jianrong, has been widely circulated on the internet in China. In it Mr Yu describes the emergence in recent years of a new type of social unrest, which he calls “venting incidents”: brief, unorganised outbursts of public rage against the authorities or the wealthy. China's efforts to enforce “rigid stability”, he argues, were not sustainable and could result in “massive social catastrophe”. Even government officials, he notes, are giving warning in private of worse to come.