Hong Kong’s national-security police arrested five people for allegedly conspiring to commit sedition through a series of picture books that portray sheep being targeted by wolves—an allusion to China’s crackdown on pro-democracy supporters in the city.

Hours after police detained five members of a speech therapists’ union, police displayed three illustrated books that they say incited hatred against the government among children as young as four. The cartoons simplified “political issues that kids wouldn’t comprehend and beautifies criminal activities,” Superintendent Steve Li Kwai-wah told a news conference. “They’re meant to poison the minds of children,” he said.

Described as teaching aids, the books were distributed through pro-democracy businesses, local political offices and online by the speech therapists’ union, which was founded in November 2019—a time when some activists formed workers’ groups as a way to organize protest actions against the government.

The books include one titled “The Guardians of Sheep Village,” which is set against the backdrop of antigovernment protests that rocked Hong Kong in 2019. It depicts a malicious plot by the wolves to take over the sheep’s village and devour them all.

Another, “12 Warriors of Sheep Village,” refers to a dozen activists who were caught by the Chinese coast guard during an ill-fated boat escape from Hong Kong last year. The third book in the series, titled “Street Cleaners of Sheep Village,” alludes to a medical workers’ strike last year when Hong Kong faced its first coronavirus infections imported from China, using cartoons of littering wolves to portray outsiders.

Police displayed evidence that they say incited hatred against the government.

Police displayed evidence that they say incited hatred against the government.

Photo: daniel suen/Agence France-Presse/Getty Images

Thursday’s arrests are part of an intensifying crackdown on dissent in the former British colony and were made on the same day that four former executives and journalists of pro-democracy newspaper Apple Daily appeared in court charged with violating the national-security law by conspiring to collude with external forces. Apple Daily, founded by jailed media mogul Jimmy Lai, was forced to cease publication last month after authorities seized its assets.

Publishers have been among the targets of authorities since the national-security law was imposed last year. Media groups and opposition groups have raised concerns that free speech is being eliminated and so-called red lines about what amounts to a crime are being expanded to eliminate criticism of authorities.

“Even children’s picture books cross the red line,” Herbert Chow, a local businessman who supports the protest movement, wrote in a Facebook post referring to the arrests.

The five people arrested—two men and three women, aged between 25 and 28 years old—are board members of the General Union of Hong Kong Speech Therapists. They were detained under a colonial-era antisedition law rather than the security law imposed by China.

In its online mission statement, the union says it has chosen to align itself with the politically marginalized. “We are a group of speech therapists, we should walk with the unheard,” it said on its website. “Those who are lucky won’t understand that being able to speak is a luxury. But we resonate with this.”

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Police said they seized around 550 related publications during their search of the suspects’ homes and offices, along with many pro-democracy fliers and protest-related figurines.

Earlier this year, Christine Choi Yuk-lin, deputy head of Hong Kong’s education bureau, criticized one of the children’s books published by the speech therapists’ union for being inappropriate to use in classrooms, warning parents it was “political propaganda.”

Mr. Li, the police superintendent, said the group had ignored Ms. Choi’s warning and went on to publish more books.

—Elaine Yu contributed to this article.

Write to Joyu Wang at joyu.wang@wsj.com