The 15-year-old had jumped over a 4-foot, 7-inch barrier surrounding an outdoor exercise area for pandas on Monday afternoon while 8-year-old Gu Gu and another bear were being fed, said a man surnamed Zhang, the director of the zoo management office.
The teen startled 240-pound Gu Gu, who bit the unwanted visitor on both legs, said Zhang, who would not give his full name.
The Beijing News identified the teen as Li Xitao, citing emergency medical officials who said he was so viciously attacked that his bones were showing. Chunks of flesh were left behind in the ambulance, they said.
Gu Gu was in the news last year when he was bitten by a drunk tourist. The man jumped into the bear's pen and tried to hug him, but was bitten instead. The tourist retaliated by biting the bear in the back.
Pandas, which generally have a public image as cute creatures, are nonetheless wild animals that can be violent when provoked or startled.
Li, who is from Hebei province in northeast China but makes a living in Beijing selling recyclables that he picks off the street, had climbed into the panda enclosure out of curiosity, the newspaper said, citing zoo security who had spoken with the boy.
Keepers who were in the exercise area at the time broke up the attack, which lasted two or three minutes, Zhang said. They managed to lure Gu Gu away from Li, Zhang said, explaining that the pandas have been trained to obey keepers' commands.
The panda enclosure operated normally after the attack, the official said. Gu Gu was seen snacking on bamboo later in the afternoon, The Beijing News said.
Zoo officials were considering unspecified measures to prevent further incidents. "We have to prepare against such behavior," Zhang said, adding that visitors should "act properly when visiting and love the animals."
The barrier around the exercise area can't be any taller because it would block the view of the pandas, he said.
Li was taken to Beijing Children's Hospital. An operator at the main number for the hospital directed calls Tuesday to the surgery department, where phones were not answered.
Police were investigating, and it was not yet clear whether Li would face any criminal charges, Zhang said.