A few days ago, Chinese police knocked on Yan's apartment door, saying they were there for a routine document check.

It didn't take long to reveal their real intention. "They asked me what I posted on Weibo," said Yan, 19, a student and amateur coder who spoke on condition his real name and other identifying details were not published.

Weibo is roughly China's Twitter, and Yan admitted to the officers that he had built a digital tool that allowed internet users to reconnect with people whose social media accounts had been deleted. He then released it amid the flurry of censorship that sought to scrub the Chinese internet of critical commentary after the Communist Party said it planned to remove term limits for the presidency, clearing the way for Xi Jinping to remain in office for life – a decision written into the national constitution Sunday.

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"I told police that I made it with no evil intent. I just wanted to find my friends," Yan said.

"But it seems I overestimated this nation's tolerance."

Indeed, what happened to Yan is among the more vivid recent examples of the intense effort Chinese authorities have made in cleansing the country of anything but support for Mr. Xi.

Police seized Yan's computer and took him to a police station where they ordered him to sign a document pledging to delete the tool and refrain from making another – and then drew his blood. "They just told me they needed to collect my DNA," he said. The officers recorded numerous personal details, and "I also saw them marking a box saying 'person of key concern,'" Yan said.

Outside China, censorship around the plan to abolish presidential term limits has attracted snickering attention. At one point, even the letter "n" (a reference to n as a mathematical representation of an unknown number) was blocked online in some places, as were words like "Winnie the Pooh," a reference to Mr. Xi's stout figure.

All of it came after an extraordinary outpouring of criticism online from people furious at Mr. Xi's plan to tear up convention and crown himself a new "emperor."

But for Yan and others, the consequences have been more serious in a country that has sought to smooth the way for a long-term leader in part by cracking down on those who dare question Mr. Xi or tarnish his state-media-crafted image of humble perfection.

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Two women in Wuhan, Huang Fangmei and Geng Caiwen, were detained, according to the The Network of Chinese Human Rights Defenders. Ms. Huang had uploaded a video of her cheerily chanting "qing zhuyi daoche!", a warning that a vehicle is backing up – and, in this case, a reference to China sliding backwards.

"It's a great example of the creativity and resilience of Chinese activists in the face of such a depressing announcement. It also shows how skittish police are," said Frances Eve, a researcher with the network.

"It's unlikely that human-rights activists are going to stop mocking Xi Jinping, but it does appear that officials are treating it as a crime as they try and protect his cult of personalty," she added.

In the past few years, students and artists have been detained for infractions such as wearing a shirt calling Mr. Xi "Xitler," a reference to Hitler; naming the president "steam bun Xi"; and displaying a "China doesn't need Xi Jinping" sign. Journalists have been disciplined, suspended and even fired for typos about Mr. Xi. Censors sprang into action when a television presenter mispronounced a single tone in Mr. Xi's name, going so far as to use copyright claims to convince YouTube to delete video of the error.

Then, after state media described the proposed constitutional term-limit change, censors rapidly moved to censor more than 50 terms, according to a list compiled by China Digital Times.

"A lot of the blocked terms are puns or historical or cultural allusions, the sort of indirect speech that we often see people adopt to comment on sensitive political topics," said deputy editor Samuel Wade.

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"The number of reported blocks reflects the huge level of attention this is receiving from users, as well as greater than usual sensitivity on the part of the censors," he added.

"One of the responses that really struck me, though, was a post whose author said that the situation was so serious that they no longer felt they could do that: 'Because this is intolerable, the furthest extreme of intolerable, I will no longer be silent. I will not satirize or use sarcasm. I will not complain. I will not use metaphor. I will clearly express my point of view.'"

On Feb. 25, the day of the announcement, censorship on Weibo reached some of its highest levels in recent years, according to Weiboscope, a tracking tool developed by the Journalism and Media Studies Centre at The University of Hong Kong.

But by the next day, it had receded nearly to normal levels. "The main reason is because a lot of the accounts that posted negative comments on the 25th were all removed," said King-wa Fu, a scholar at the centre.

It was for that reason that Yan developed his tool, placing it on a website that attracted 240,000 views in its first 12 hours.

"After news about the presidency went viral, many people openly commented before their accounts were quickly spotted and terminated," he said.

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He likened the effect to a bomb being detonated in a jammed stadium. "People cried, shouted to find their missing friends and actively rescued the wounded."

"I can't tell you how many people in my friend circle were bombed, there are so, so many. I can't count it."

His tool was meant to give people a way to find each other.

But police told him he had brought detention upon himself, by circumventing censors intent on driving offenders into isolation.

Yan first bumped into China's internet censorship in his early high school years, when he tried to play online games that were blocked. A self-described otaku, he built his own tools to circumvent what is called the Great Firewall, trading access to friends who gave him money for cigarettes.

But after his recent detention, which lasted into the evening, he isn't certain what to do. Police eventually gave him back his computer, but not before a friend suggested he stow away on a boat to Taiwan.

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"I found that absurd – I didn't do anything wrong but end up being a political prisoner?" he said.

"My dream is to see this country peacefully evolve. But I know without any doubt I can do nothing about that."

– with reporting by Alexandra Li