When the engineer did not comply after 24 hours, he discovered that someone had hacked into his Twitter account and deleted its entire history of 11,000 tweets.

“If the authorities hack you, what can you do?” said the engineer, who spoke on the condition of anonymity for fear of landing in deeper trouble with authorities. “I felt completely drained.”

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In Beijing and other cities across China, prominent Twitter users confirmed in interviews to The Washington Post that authorities are sharply escalating the Twitter crackdown. It suggests a wave of new and more aggressive tactics by state censors and cyber-watchers trying to control the Internet.

Twitter is banned in China — as are other non-Chinese sites such as Facebook, YouTube and Instagram. But they are accessed by workarounds such as a virtual private network, or VPN, which is software that bypasses state-imposed firewalls.

While Chinese authorities block almost all foreign social media sites, they rarely have taken direct action against citizens who use them, preferring instead to quietly monitor what the Chinese are saying.

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But recently, Internet monitors and activists have tallied at least 40 cases of Chinese authorities pressuring users to delete tweets through a decidedly low-tech method: showing up at their doorsteps.

Even for a country accustomed to censorship, a crackdown on Twitter is surprising because the service, like Google and Facebook, is used by a relatively small number of people, at least by Chinese standards.

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An estimated 10 million Chinese use Twitter, according to some tech-industry watchers. (Twitter does not issue statistics on China.) That is a minuscule figure compared with those on government-approved messaging and app sites: 1 billion for WeChat and hundreds of millions on Weibo, according to state figures.

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But in the past two years, as the space for political speech has all but vanished in President Xi Jinping’s China, Twitter has played an increased role. It has become a cyber-window to the outside world, a release valve for the disaffected, a virtual teahouse for politically minded Chinese at home and abroad.

Bankrupt mom-and-pop investors fume about the lack of financial regulations. Disgruntled farmers pass around videos of land seizures or police thuggery. Muslims from China’s far west share pictures of loved ones locked away in state-operated reeducation centers.

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It has started to resemble the freewheeling Twitterscape in other tightly controlled nations such as Saudi Arabia and Egypt.

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And to the Chinese Communist Party, that means it is a rising threat.

“Twitter is the fastest, simplest, most important gathering place if you care about Chinese politics. It’s extremely hot right now,” said Ho Pin, the New York-based publisher of the Mirredia Group, a leading purveyor of sensitive Chinese political news.

In late November, the wife of renowned photographer Lu Guang took to Twitter to seek help for her missing husband, believed to be detained by police. (His name is censored on the Weibo service.)

Last summer, when the Chinese government tried to break up a nationwide labor movement, tech-savvy student supporters informed the world via Twitter.

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An elite class of businesspeople with ties to the upper echelons of the Communist Party as well as media professionals are increasingly sneaking peeks at the banned service, according to Ho.

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“They all read it,” he said. “For the government, the threat exceeds that of anything else. Twitter has become their biggest target to take down.”

That’s precisely what’s happening now.

He Jiangbing, a financial commentator, said police came to his Beijing living room to warn about his tweets.

Days earlier, officials visited the Guangzhou home of Ye Du, a well-known writer and supporter of the late Nobel Peace Prize laureate Liu Xiaobo, to hand him a printout of 802 tweets he needed to delete, Ye said in an interview.

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Meanwhile, all 30,000 tweets from the account of Wu Gan, an activist serving an eight-year prison sentence, were deleted in November, which suggested a government hack, said Yaxue Cao, a Washington-based activist.

Cui Haoxin, a Muslim poet, was taken to a police station and interrogated last week — partly because of his tweets calling attention to Islamophobia in China.

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“They not only violated my personal freedom of speech, but they’re effectively violating a foreign company and a foreign country’s Internet sovereignty,” Cui said.

Interest among Chinese users skyrocketed in early 2017 after the fugitive billionaire Guo Wengui began using Twitter and YouTube to air sensational — and largely unsubstantiated — allegations of corruption against Chinese leaders.

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The campaign would have been unthinkable on Chinese social media. But it played out dramatically over several months on Twitter, drawing in a new generation of Chinese users.

For months, Guo and his supporters shared corporate records and satellite images of California mansions allegedly linked to senior Communist Party officials.

“We tend to see cycles of tightening and relaxing, and China is clearly in a tightening phase currently as the economy slows down and domestic and intentional challenges loom large,” said Dali Yang, an expert on Chinese politics.

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The Ministry of Public Security and the Cyberspace Administration of China, the Internet regulator, did not respond to requests for comment.

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The first time Chinese state security actively sought out Twitter users was in 2011, when Chinese dissidents, inspired by the wave of uprisings in the Arab world, tried to use the platform to mobilize protests, said Yaqiu Wang, a China researcher at Human Rights Watch.

After that movement fizzled, Chinese authorities have kept an eye on the platform but, until recently, rarely intervened. That helped cultivate Twitter’s reputation as a safe space for an improbable and colorful cast of Chinese voices.

The veteran journalist Gao Yu, for one, tweets prodigiously despite living under house arrest after being charged with leaking state secrets in 2015. Bao Tong, a purged 86-year-old Communist Party official and former top aide to Chinese leader Zhao Ziyang, set up an account last year and hurls daily criticism of the Xi administration to 137,000 followers. (Calls by foreign journalists to Bao’s home are frequently cut off, but his access to Twitter appears to be rarely interrupted.)

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He, the Beijing-based columnist, said there are fears that a full-scale crackdown on Twitter would choke off the last online venue in China for open intellectual debate.

Last year, He wrote pointed comments on Weibo and WeChat urging the Chinese central bank to lower reserve requirements for lenders because, in his estimation, the Chinese economy was struggling.

“I never touched politics. I’m not a dissident, and I’m not a celebrity,” he said. “They still took away my voice.”

When police visited him last month to warn him about tweeting, He did not promise to stop. He’s not sure he could.

“We go to Twitter because we have no choice,” He said. “For people like us, if you have to hold in your thoughts, it feels like dying.”