Twenty pupils at a Beijing primary school were wounded on Tuesday in an attack that police said had left at least three seriously injured, state news agency Xinhua reported.
Three children suffered serious but non-life-threatening injuries, the Xicheng district government said on its social media account.
All of the injured children are now being treated at a local hospital.
The Xicheng government did not give other details of the attack, but posts on social media said the children suffered knife wounds to the head.
The man is a maintenance worker at the school and his contract is due to expire at the end of January, police said.
The attack happened at Xuanwu Normal Experimental Affiliated Number One Primary School just after 11.15am.
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A video circulating online showed the school's headmaster telling angry parents that the attack occurred on the first floor, which held Year Two classes.
"The children were attacked while they were doing exercises on the school running track".
Earlier in a WeChat discussion one parent claimed: "The attacker. used a hammer-like weapon to hit the children's skulls one by one, before he was stopped by the PE teacher". According to the school's website, it has 2,537 students and 199 teaching staff.
Jia Liquan, 49, whose work with the school ended this month leaving him jobless, is understood to have attacked the children in the playground during break time. In October last year, a 39-year-old woman was detained on suspicion of stabbing 14 children at a kindergarten in Chongqing.
In November, five people were killed when a vehicle ploughed into a crowd of children outside a north-eastern Chinese nursery.