The Chinese professor Xu Zhangrun, who published a rare public critique of President Xi Jinping over China’s coronavirus crisis, was placed under house arrest for days, barred from social media and is now cut off from the internet, his friends have told the Guardian.

Xu’s passionate attack on the government’s system of controls and censorship, Viral Alarm: When Fury Overcomes Fear, was published this month – a rare, bold expression of dissent from the liberal camp under Xi’s rule.

A friend of Xu’s who spoke on Sunday on the condition of anonymity to avoid reprisals said police placed Xu under house arrest soon after he returned to Beijing from his lunar new year break at his home town in Anhui province.

“They confined him at home under the pretext that he had to be quarantined after the trip,” the friend said. “He was in fact under de facto house arrest and his movements were restricted.”

During those days, at least two people stood guard in front of his house around the clock and a car with a signal box was parked in front of his residence. Security agents also went into his house to issue warnings to him, the friend said.

Those restrictions were lifted late last week, but his internet connection has been cut off since Friday, the friend added.

“He tried to get it mended but found out that his IP [internet protocol address] has been blocked. He lives on the outskirts of Beijing and is far away from shops and other services. Under the current [coronavirus] situation, things are very difficult for him.”

Friends say that since publication, Xu’s account has been suspended on WeChat, a Chinese messaging app, and many have been unable to get in touch with him for days. His name has been scrubbed from Weibo, a Twitter-like microblog, with only articles from official websites several years ago showing up on the country’s biggest search engine, Baidu. Calls to his mobile phone went unanswered on Sunday.

Phone calls to the Ministry of Public Security also went unanswered on Sunday. The staff member who answered the phone at Changping branch of Beijing Public Security Bureau said she had no knowledge of Xu.

Another friend who also spoke on the condition of anonymity had managed to correspond with him through text messages but said his situation was worrying. “I fear he might be under surveillance,” said this friend. “He has not directly responded (to my queries) but just told me not to worry.”

When Xu published his essay, he warned that he was likely to be punished. He said he had already been suspended from teaching and had “freedoms curtailed” over critiques published nearly a year earlier.

“I can now all too easily predict that I will be subjected to new punishments; indeed, this may well even be the last piece I write,” he wrote at the end of his latest essay.

Xu’s criticism of the country’s leadership came shortly before a widespread debate on freedom of speech convulsed the country. The death on 7 February of whistleblowing doctor Li Wenliang, who had tried to warn colleagues about the virus but was reprimanded and silenced by security forces, triggered an outpouring of grief and anger and an unusual public discussion about censorship.

“Li’s death has thoroughly exposed the ills of the party’s governance and control; this has a huge impact on people’s minds,” said Hong Zhenkuai, an independent historian who is currently working outside China, as a visiting scholar at Tokyo University.

The mechanisms that normally constrain Chinese journalists have also eased slightly, with some of the most powerful stories about life in quarantined Wuhan and the latest news about the evolution of the outbreak coming from mainland newsrooms like that of magazine Caixin.

But public anger over censorship, and the particular circumstances of a national emergency, should not be mistaken for any fundamental change within the Chinese Communist party, which has been honing its ability to control the national conversation for decades, activists and intellectuals say.

In a further reminder of the government’s strict controls, two citizen journalists who were reporting from the epicentre of China’s coronavirus outbreak have vanished this week, apparently detained.

The Chinese military surgeon who exposed the government’s cover-up of the Sars outbreak in 2002-2003 has been under de facto house arrest since last year, the Guardian revealed this month. Detention came after he wrote to the top leadership asking for a reassessment of the 1989 Tiananmen Square pro-democracy movement.

“There is no space for speech freedom in China now,” said Hong. “The impacts on the individuals are multi-faceted. Economically, they would cut off your livelihood [academics get fired, writers can’t publish and no one dares hire you]. You would get sidelined by mainstream society, you’d lose friends and, worse than that, you might lose your personal freedoms, so a number of intellectual elites have chosen to leave China.”

Since he took power in late 2012, Xi has tightened ideological control and suppressed civil freedoms across the nation, reversing a trend under his predecessor to give Chinese media some limited scope to expose and report regional corruption and lower-level officials’ misdeeds.

Even within the Communist party, cadres are threatened with disciplinary action for expressing opinions that differ from the leadership.

Under Xi’s crackdown on speech and academic freedoms, a number of prominent liberal intellectuals, journalists, rights lawyers and NGO workers have either been silenced, jailed or escaped abroad.