BEIJING (REUTERS) - A Chinese man attacked at least 20 young children at a Beijing primary school with a hammer on Tuesday (Jan 8), state television reported, in a rare act of violence in the capital.

A 49-year-old man - a former maintenance worker at the school - was detained after venting his anger on the children after his work contract was not renewed, state television cited police as saying.

Twenty were taken to hospital, the government in the Xicheng district of the capital said in a post on its social media account.

Three suffered serious injuries but their conditions were stable, it said.

Police cars lined the main road close to the school. Several police were seen coming from the school, carrying items including sealed paper bags marked "physical evidence" and silver cases before driving off.

Parents waited outside for their children as the school day ended.

"I heard that children were attacked (by someone) with a knife, so I'm very anxious," said a woman surnamed Zhou as she waited for her child to finish school.

Another parent, surnamed Jia, said she was concerned about her child's psychological well-being.

"Even though they were on the same floor as the one where this took place, they didn't know anything about this. So I really don't want this information to be spread widely so that he starts to feel scared," she said.

The attacker, originally from north-eastern Heilongjiang province, had been detained by police, state television said.

There has been a series of knife and axe attacks in recent years, many targeting children.

In January 2017, a man in southern Guangxi Zhuang autonomous region stabbed and wounded 12 children with a kitchen knife. He was executed this month.

"People who hurt children do not deserve to be forgiven," a social media user wrote in a post on Weibo, a Twitter-like microblog.

Last April, a 28-year-old man killed nine middle-school students as they were returning home in one of the country's deadliest knife attacks in recent years.

The killer, who said he had been bullied when he attended the school in northern China's Shaanxi province, was executed in September.

Later, in 2018, a knife-wielding woman injured 14 children at a kindergarten in south-western Sichuan province.