A 29-year-old man caught beating a toddler mercilessly on a surveillance camera has been detained by local police after the video appeared online and stirred widespread anger.

The footage shows a boy playing with a broom outside a shop before a man comes and savagely beats him for 37 seconds in Luochuan county, Yan'an of Northwest China's Shaanxi province on May 4.

The man first kicked him to the ground, stamped on him repeatedly and then used the broom and dustpan to repeatedly hit him. Two people passed by in the early stages of the assault but did nothing. The boy was eventually rescued by bystanders who ran out of the shop and stopped the attack.

"At first I thought it was a father disciplining his child," Wang Guoqing, a 25-year-old eyewitness, the first man appearing on the footage to stop the attacker, told the Chengdu Business Review.

"But how could a father be so cruel? It seemed like he didn't care whether the child lived or died. He is crazy."

The victim, who is two years old, suffered a skull fracture and several soft tissue injuries, but is now out of danger. He was discharged from hospital on May 7, said the official micro-blog of China Central Television (CCTV).

Wang Jinlong, the man who carried out the beating, has been diagnosed with schizophrenia and detained, according to local police. He had reportedly beaten a woman until she sustained fractures before laying his hands on the toddler.

The video, which appeared online on Thursday, has generated tens of thousands of comments on Sina Weibo, China's Twitter-like social networking site, in one day, with many expressing outrage over the attack and the indifference of the two passers-by.