Twenty children were taken to hospital, the government in the Xicheng district of Beijing said on its social media account. All the children were taken to the hospital and expected to survive.
In a statement, Xicheng district police announced that they had captured the attacker at the scene.
Jia, a native of northern Heilongjiang province, was employed through a labour service company to perform daily maintenance work at the school. His contract was set to expire this month and had not been renewed.
Authorities did not say what weapon, if any, was used.
Parents outside the school.
The attack which took place at the Beijing No.1 Affliated Elementary School of Xuanwu Normal School, was carried out with a hammer, according to state-media outlet the Global Times.
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"The details are still not clear", wrote one parent on WeChat. According to the school's website, it has 2,537 students and 199 teaching staff.
Such incidents have forced authorities to increase security around schools and led to calls for more research into the root causes of such acts.
"They were all younger students and were stabbed by an adult".
People leaving the school.
Last October, a 39-year-old woman was arrested after hurting 14 children in a knife attack in Chongqing, a city in southwestern China, as students returned to class after morning exercises outside.
In November, a driver rammed his vehicle into a group of children crossing a street in front of an elementary school in northeastern Liaoning province, killing five people and injuring at least 19.