His nickname in China was “The Cannon,” and Ren Zhiqiang’s latest commentary was among his most explosive yet.

Mr. Ren, an outspoken property tycoon in Beijing, wrote in a scathing essay that China’s leader, Xi Jinping, was a power-hungry “clown.” He said the ruling Communist Party’s strict limits on free speech had exacerbated the coronavirus epidemic.

Now Mr. Ren, one of the most prominent critics of Mr. Xi in mainland China, is missing, his friends said on Saturday.

His disappearance comes amid a far-reaching campaign by the party to quash criticism of its slow, secretive initial response to the epidemic, which has killed over 3,100 people in China and sickened more than 80,000.

The Chinese government is working to portray Mr. Xi as a hero who is leading the country to victory in a “people’s war” against the virus. But officials are contending with deep anger from the Chinese public, with many people still seething over the government’s early efforts to conceal the crisis.

Mr. Ren, a party member, is well known for his searing critiques of Mr. Xi. In 2016, the party placed him on a year’s probation for denouncing Mr. Xi’s propaganda policies in comments online.

The government has monitored Mr. Ren’s movements intensely ever since, friends said, preventing him from leaving the country and deleting his social media accounts, where he had built a wide following.

His whereabouts was unclear on Saturday, and the police in Beijing did not immediately respond to a request for comment.

“We’re very worried about him,” said Wang Ying, a retired entrepreneur and friend of Mr. Ren’s. “I will continue to look for him.”

In recent weeks, an essay by Mr. Ren began circulating among elite circles in China and abroad. In it, he blamed the government for silencing whistle-blowers and trying to conceal the outbreak, which began in the central city of Wuhan in December.

While he did not explicitly use Mr. Xi’s name in the commentary, Mr. Ren left no doubt he was speaking about China’s leader, repeatedly referencing Mr. Xi’s speeches and actions.

“I see not an emperor standing there exhibiting his ‘new clothes,’ but a clown who stripped naked and insisted on continuing to be an emperor,” he wrote.

Addressing Mr. Xi, he wrote: “You don’t in the slightest hide your resolute ambition to be an emperor and your determination to destroy anyone who won’t let you.”

Mr. Ren, 69, is the retired chairman of Huayuan Properties, a real estate developer. In 2016, Mr. Ren came under scrutiny after writing on his microblog that China’s news media should serve the people, not the party, contradicting one of Mr. Xi’s high-profile pronouncements. His remarks offered a window into growing frustration among Chinese intellectuals and entrepreneurs over Mr. Xi’s increasingly authoritarian rule.

The party moved quickly to censure him, saying he had “lost his party spirit.” But he continued to speak out on other topics, such as China’s strict policies to limit the population in big cities.

As more details about China’s efforts to cover up the coronavirus outbreak have been disclosed by the Chinese news media in recent weeks, Mr. Xi has come under attack from several prominent Chinese activists and intellectuals.

Xu Zhangrun, a law professor in Beijing, published an essay last month saying that the epidemic had “revealed the rotten core of Chinese governance.”

Xu Zhiyong, a prominent legal activist, released a letter to Mr. Xi on social media, accusing him of a cover-up and calling on him to step down. He was later detained.

Activists said Mr. Ren’s disappearance was a worrying sign that the government was escalating its latest crackdown on free speech.

“The epidemic has brought out the worst of Xi Jinping,” said Yang Jianli, a rights activist based in the United States. “He is so determined not to give an inch, rightly understanding an inch would mean hundreds of miles.”

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