Two teachers have been detained in China, local police said, after a picture posted online showing one of them lifting a screaming boy by his ears sparked widespread anger.

The teacher who is alleged to have held the boy, in the most recent of a series of violent acts against schoolchildren in China, told state media that she did it "for fun" after asking her colleague to take the picture.

"She didn't approve of what I was doing initially, but I told her that it was just for fun and would take just a second," the woman, who was named as Yan Yanhong, told the website of magazine China Newsweek.

Netizens in China reacted with fury to the image, which was taken at the Blue Peacock Kindergarten in Wenling in China's eastern Zhejiang province, and posted online on Wednesday.

The incident quickly become a hot topic on Weibo, China's version of Twitter, and remained the second most talked about issue on Friday with more than four million messages.

"When you look at yourself picking up this child with a frightened expression of pain, do you not feel bad?" said one poster.

"This is obviously child abuse, but it is not a crime under the existing criminal law and she cannot be punished," said another.

Yan, who was said by state news agency Xinhua to be a 20-year-old unlicensed teacher, was detained on Thursday for "criminal behaviour", Wenling police said in an online statement.

The teacher who took the pictures has been detained for seven days, the statement added.

A commentary by state-run news agency Xinhua blamed the lack of investment in nurseries for the incident.

"Similar scandals have been reported in increasing numbers in recent years, exposing a lack of supervision and public investment in preschool education," the commentary said.

Child welfare is becoming an increasingly important issue in China.

The nation was shocked last year when a two-year-old girl died when she was run over by a hit-and-run driver.

Surveillance camera footage showed at least 18 people walk past the girl after she was knocked down on a street in the in the southern city of Foshan, in Guangdong province.