The booth's owner fled and is wanted for questioning.

The raid came after an investigative TV reporter uncovered the dodgy buns in a kitchen a few days earlier. The kitchen was used to prepare the dumplings for later sale at the streetside booth in Beijing's Chaoyang District.

A video broadcast on Wednesday night on China Central Television Station showed an undercover interview conducted with a hidden camera.

The segment opened with a shot of cardboard piled in a heap between rows of shabby houses.

The camera followed a man, whose face was not shown, into a ramshackle building where steamers were filled with many fluffy white buns, the type traditionally stuffed with minced pork.

The shirtless, shorts-clad man, believed to be the owner, apparently thought the reporter was a wholesale customer for the buns.

When the reporter asked why cardboard filler was being used, the interview subject said it was done to lower costs.

The man and a woman in the house then showed the reporter how the process worked.

Cardboard was soaked in water, and an industrial-use caustic soda, a poisonous chemical, was added. The cardboard lost its normal color, became softer and started to look more like pork.

"Can customers recognize the cardboard?" the reported asked.

The man replied, "Most of them can't, as pork fat is stirred into the concoction to make the stuffing taste more authentic."

When asked the proportion of the raw materials, the man said the mix was 60 percent cardboard to 40 percent pork fat.

About 10 minutes later, steaming servings of the buns appeared on screen. The reporter took a bite.

"This baozi filling is kind of tough. Not much taste," the reporter said. "Do you eat them?"

The man answered, "No."

"Most of my customers are residents in nearby areas," the man said. "It may save me almost 1,000 yuan (US$132) a day."

It was unclear how long the booth had been serving the cardboard-filled dumplings. The kitchen was in nearby Taiyanggong Village, far enough away that customers couldn't discover the true nature of the dumpling ingredients.

Officials with the Zuojiazhuang Industrial and Commercial Administration closed down the kitchen yesterday and began questioning its landlord, according to the Beijing Times report.

Chaoyang District's Industrial and Commercial Administration said it will inspect the district's 58 dim-sum restaurants to make sure similar shortcuts aren't being taken.

Pork prices in 36 major cities across the nation continued to rise last month due to a continuing supply shortage.