sweeping national security law passed on June 30 instantly altered the lives and liberties of Hong Kong’s residents, criminalizing words and images that just hours earlier had been legally protected free speech.

The next day, thousands of pro-democracy demonstrators tested the limits of the new law. Some carried signs bearing slogans like these, which for months had been lawfully displayed in the streets of the semiautonomous Chinese city.

The police have since arrested more than 20 people under the new law, which lays out political crimes punishable by life imprisonment in serious cases, and allows Beijing to intervene directly if it wants.

Hong Kong was once a bastion of free speech. It served as a base for the international news media and for rights groups, and as a haven for political refugees, including the student leaders of the 1989 Tiananmen Square protests in Beijing. Books on sensitive political topics that are banned in mainland China found a home in the city’s bookstores.

But the limits of the security law are vaguely defined. As a result, artists, journalists, activists, academics and others risk running afoul of the law for what they say, write, or tweet.

The owners of this bubble tea shop, who had earlier publicly supported the protests, removed the pro-democracy ephemera that once decorated their store.

Passages about corrupt party officials and the 1989 Tiananmen Square crackdown on democracy protesters, a topic largely taboo in schools in the mainland, have been amended or removed from new textbooks, a Times analysis has found.

The enforcement of the new security law in Hong Kong’s schools and universities targets the city’s younger residents, who played a critical role in months of protests last year. Forty percent of the 10,000 protesters arrested over the past year were students and about one in six were under the age of 18, according to the police.

Libraries have removed books written by democracy activists and placed them under review. And writers working on sensitive topics have sought publishers overseas.

Raymond Yeung, the author of “To Freedom: A Year of Defiance in Hong Kong,” said three printers in the city refused to produce his book after the law was passed.

To get the book published, he said, he had to remove photos that included the slogan “Liberate Hong Kong” and all mentions of independence for Hong Kong.

The security law has also sent a chill through Hong Kong’s once freewheeling news media.

RTHK, the public broadcaster, removed a political podcast from its website after the authorities warned that an interview with Nathan Law, a democracy activist now living abroad, could be in breach of the new law.

In August, Jimmy Lai, the publisher of Apple Daily, a local newspaper, was arrested under the law. During a raid at the office of Mr. Lai’s newspaper, the police selectively barred several news outlets from getting past their cordon.

Lau Kwong Shing, an illustrator known for artwork supporting the protests, said he planned to leave Hong Kong, but in the meantime would take a break from explicitly political drawings.

“Staying in Hong Kong could become dangerous,” Mr. Lau said. “What I illustrate is just an expression of my thoughts, but that might now come with legal consequences.”

Others have sought creative ways to skirt the law. They carry blank signs or ones with coded messages. They play protest songs but without lyrics.

But there are concerns that even such workarounds may be deemed illegal.

“The police were giving warnings to young protesters holding blank signs,” said Claudia Mo, a pro-democracy lawmaker. “They are trying to say: ‘If we say you’re expressing a criminal opinion, then that’s it, because we are the law.’”