The incident at Lingxi city in Cangnan county in the eastern province of Zhejiang is the latest instance of public outrage triggered by the behaviour of China's "chengguan", quasi-police officials who enforce local regulations and have a reputation for brutality.
Five chengguan were injured in the protest, with two in critical condition, the state-run China Daily newspaper reported Sunday. The passer-by, a man surnamed Huang, was in stable condition.
According to an account posted by the Cangnan government on its official microblog Saturday night, the incident began when several chengguan in Lingxi demanded that a vendor stop "illegally" selling gas stoves and other items, which they said were blocking the sidewalk.
Huang, who happened to be passing by, began taking photos, and "after the officers demanded he stop, to no avail, both sides clashed," the official report said.
Huang was injured in the altercation and taken to hospital, the Cangnan government said.
The official account stated that Internet rumours about "urban management workers beating a man to death" began circulating in the afternoon, triggering a mass gathering of onlookers during which five officers were "besieged and beaten".
A report in the Southern Metropolis Daily on Sunday quoted several eyewitnesses who said Huang, 36, was beaten by more than a dozen officers, some uniformed and others in plainclothes.
One local resident told the paper that the vendor who triggered the incident was a young woman and that her stoves "were not blocking the road; she just placed them in front of the store".
After Huang declined to hand over his phone, the officers punched him to the ground, then kicked him for more than a minute until he was vomiting blood, eyewitnesses said.
The uniformed officers fled after the incident, while the plainclothes men got inside a yellow van but were soon surrounded by angry onlookers, some of whom burst the vehicle's tyres to prevent an escape, the paper reported.
It said some protesters smashed the vehicle's windows with bricks and others overturned an ambulance.
The incident follows reports earlier this month that urban management officers in the eastern city of Fuzhou beat an old man to death, an episode that triggered national outrage online.