It hardly seemed a threatening scene when, on a Friday afternoon in February, dozens of finance wonks gathered in Beijing for a three-hour symposium on China's exchange-rate mechanism. With a slide show featuring graphs and formulas, the main speaker talked about the arcana of the yuan's adoption last year as one of the IMF's reserve currencies (a development that Chinese reformists hope will encourage the government to let the yuan float freely).

Other participants sipped their green tea, jotted down notes and chipped in with their views. The host, the Unirule Institute of Economics, often conducts such events. Typically, it posts summaries of speakers' views online. Not this time, however. The website is no more.

It was shut down by the city government in late January, as was another site run by Unirule, as well as all of the institute's social-media accounts and those of its leading researchers.

Their closure was the latest blow to the country's moderate liberals, who for many years have continued to enjoy at least some freedom to debate reforms, even while the authorities have been busy rounding up more radical critics of the regime.

The onslaught against liberal forums began in July when, after a quarter-century of cheerleading for reform, including better implementation of the constitution's guarantees of protection for human rights, a monthly magazine called Yanhuang Chunqiu was taken over by hard-liners.

The magazine's founding publisher, Du Daozheng, said the purge reminded him of the Cultural Revolution, when radical Maoists seized control of a newspaper for which he then worked, and berated him as a "counterrevolutionary."

In October, a website called Consensus Net, much loved among liberals, was closed. It had been publishing articles about economic, social and political reform since its founding in 2009. With the termination of Unirule's online accounts, moderate reformists have little space left for open debate.

China's president, Xi Jinping, professes himself to be in favor of market-oriented reform and upholding constitutional rights, and his late father supported Yanhuang Chunqiu; yet he is nervous of liberal views. Unirule has been one of the country's most prominent independent think tanks since it was founded in 1993. David Kelly of China Policy, a Beijing-based consulting firm, said the institute "cannot be said to be unorthodox, subversive or dissident in any obvious way." He notes that pro-market reform measures that were proposed by Xi in 2013 echoed those suggested by Unirule years earlier.

But these are tense political times in Beijing, as the Communist Party prepares for a five-yearly congress in the autumn and a sweeping reshuffle of its leadership (not including Xi's positions) immediately after it. Xi does not want anyone to embarrass him amid the political horse-trading of the months ahead. Websites run by die-hard Maoists, dripping with criticism of free markets, are still allowed to operate. But Maoists, at least, can be counted on to support the party. Xi seems to fear that moderate liberals may rattle it.

Unirule's co-founder, Mao Yushi, has a habit of doing so. A few days before the authorities pulled the plug on his institute's accounts he had joined dozens of intellectuals in calling on China's chief justice, Zhou Qiang, to step down. Zhou had angered them by denouncing the "erroneous influence" of calls for an independent judiciary. Mao's views matter: Before it was disabled, his account on Weibo, a Twitter-like service, had 2.7 million followers.

Sheng Hong, another co-founder of the institute, said the authorities have not even bothered to notify Unirule of their action, let alone explain it. Official media said that Unirule's online accounts were among several that had been closed for a variety of infractions, ranging from the provision of unauthorized news and information services to the broadcast of pornography. They did not specify which of the websites had broken which rule, but Unirule was certainly not guilty of the latter. The curves on its website were of the economic rather than the bodily sort.

"Maybe they hate us because we tell the truth," laments Unirule's Sheng. "But we should do this in a great nation. If we don't, China will have no future."

There is scant evidence that Xi agrees.