An outspoken Chinese Communist Party critic and millionaire property tycoon, Ren Zhiqiang, has been placed under investigation for "serious violations of discipline and law", an anti-graft watchdog said.

The Beijing Commission for Discipline Inspection announced late on Tuesday that the 69-year-old former chairman of the state-owned real estate developer Beijing Huayuan Group was under investigation.

Rights campaigners accuse President Xi Jinping and the Communist Party of using charges such as "disciplinary violations" -- often considered to refer to corruption -- as a way to silence dissent.

Beijing has stepped up its crackdown on civil society since Xi took power in 2012, tightening restrictions on freedom of speech and detaining hundreds of activists and lawyers. 

Ren disappeared from the public eye in March, shortly after penning an essay that was fiercely critical of Xi's response to the coronavirus outbreak.

The retired entrepreneur has emerged in recent years as one of the Party's most prominent critics in the business world.

His essay has been scrubbed from China's internet, which regularly censors content that challenges the authorities, but it has been shared online outside China and a copy has been saved by news aggregator China Digital Times.

"This epidemic has revealed the fact that the Party and government officials only care about protecting their own interests, and the monarch only cares about protecting their interests and core position," Ren wrote, without referencing Xi by name.

It also accuses the government of concealing the initial outbreak.

Nicknamed "Big Cannon" for his fiery rhetoric, Ren formerly enjoyed close links with major figures in China's political establishment, including his former classmate Vice President Wang Qishan.

A Communist Party member for decades, Ren was also an influential blogger on the Twitter-like Weibo platform, where he had millions of followers. 

His account was closed by authorities in 2016 after he repeatedly called for greater freedom of the press.

Yaqiu Wang, China researcher at Human Rights Watch, said the probe against Ren fitted a pattern of the Communist Party using charges such as "disciplinary violations" to silence its critics.

"It is quite clear that the Chinese government is punishing Ren for his speech critical of President Xi Jinping and Chinese government under the veneer of law," Wang said. 

"The government has a record of criminalising peaceful speech using bogus charges."

According to Wang, Ren is likely to be being held in a form of extrajudicial secret detention known as "liuzhi".

"Under liuzhi, detainees are held incommunicado -– without access to lawyers or families –- for up to six months," said Wang.

Human rights lawyer Li Fangping said the announcement was deliberately timed to minimise its public impact, coming just hours before travel restrictions were lifted in Wuhan, the city at the centre of the coronavirus epidemic.