SHANGHAI — China has largely blocked the WhatsApp messaging app, the latest move by Beijing to step up surveillance ahead of a big Communist Party gathering next month.

The disabling in mainland China of the Facebook-owned app is a setback for the social media giant, whose chief executive, Mark Zuckerberg, has been pushing to re-enter the Chinese market, and has been studying the Chinese language intensively. WhatsApp was the last of Facebook products to still be available in mainland China; the company’s main social media service has been blocked in China since 2009, and its Instagram image-sharing app is also unavailable.

In mid-July, Chinese censors began blocking video chats and the sending of photographs and other files using WhatsApp, and they stopped many voice chats, as well. But most text messages on the app continued to go through normally. The restrictions on video, audio chats and file sharing were at least temporarily lifted after a few weeks.

WhatsApp now appears to have been broadly disrupted in China, even for text messages, Nadim Kobeissi, an applied cryptographer at Symbolic Software, a Paris-based research start-up, said on Monday. The blocking of WhatsApp text messages suggests that China’s censors may have developed specialized software to interfere with such messages, which rely on an encryption technology that is used by few services other than WhatsApp, he said.

“This is not the typical technical method in which the Chinese government censors something,” Mr. Kobeissi said. He added that his company’s automated monitors had begun detecting disruptions of WhatsApp in China on Wednesday, and that by Monday the blocking efforts were comprehensive.

Facebook declined to comment, following past practice when asked about WhatsApp’s difficulties in China.

Lokman Tsui, an internet communications specialist at the Chinese University of Hong Kong, said that WhatsApp seemed to have been severely disrupted starting on Sunday. But he said that some WhatsApp users might still be able to use the service.

The Chinese authorities have a history of mostly, but not entirely, blocking internet services, as well as slowing them down so much that they become useless. The censorship has prompted many in China to switch to communications methods that function smoothly and quickly but that are easily monitored by the Chinese authorities, like the WeChat app of the Chinese internet company Tencent, which is based in Shenzhen.

“If you’re only allowed to drive one mile per hour, you’re not going to drive on that road, even if it’s not technically blocked,” Mr. Tsui said.

The disruption of WhatsApp comes as Beijing prepares for the Communist Party’s congress, which starts Oct. 18. Held once every five years, the congress chooses the party’s leadership, which in turn runs the country. Next month’s meeting is expected to reconfirm President Xi Jinping’s nearly absolute grip on power, but considerable uncertainty remains over who will join him on the Standing Committee of the Politburo, the party’s highest-ranking group.

Over the past several years, China has not only stepped up censorship but also closed numerous churches and jailed large numbers of human rights activists, lawyers and advocates for ethnic minorities.

The shutdown of WhatsApp prompted considerable dismay on Chinese social media.

“Losing contact with my clients, forced back to the age of telephone and email for work now,” one user complained on Weibo, a Twitter-like microblogging site.

“Even WhatsApp is blocked now? I’m going to be out of business soon,” another Chinese social media user said on Weibo.

In China, even the use of email is fading as residents embrace the convenience of WeChat. The messaging service, which has 963 million active users, bears some similarities to WhatsApp but has a wider array of features and one crucial difference: close ties to the government. This month, WeChat sent a notice to users reminding them that it complied with official requests for information.

When China joined the World Trade Organization in 2001, it agreed to open online data services and other enhanced telecommunications services to international competition. But it obtained the assent of other W.T.O. members to retain restrictions on the media. Technology multinationals, heavily dependent on the Chinese market, have been reluctant to accuse Beijing of falling short of its W.T.O. commitments.

The Office of the United States Trade Representative has opened a formal investigation into whether China is violating the intellectual property of American companies, but it has released few details. The trade office has not said whether the inquiry will include the blocking of products that rely on American intellectual property, or whether it will focus more narrowly on cases in which China has allegedly purloined or otherwise copied it.

WhatsApp has a strong reputation among cryptographers for security, which may have been what drew the attention of Chinese censors. The app provides so-called end-to-end encryption, which effectively means that even Facebook does not know what is being said in the text, voice and video conversations passing through its servers.

WhatsApp’s video, file sharing and other advanced features rely partly on broadly used internet data transfer protocols, and were disrupted over the summer. But its text messaging function relies on a different, heavily encrypted method for moving data that has seldom been used by companies.

The latest disruption of the WhatsApp messaging system suggests that China’s censorship apparatus may have figured out how to target the more uncommon and heavily encrypted data transport protocol as well, Mr. Kobeissi said.

Other services provided by American technology companies are available in mainland China. The country tolerates Microsoft’s Skype service for phone calls, which does not provide end-to-end encryption and as a result is easier for governments to monitor. Beijing also allows Apple’s FaceTime service, which has end-to-end encryption but does not have a WhatsApp-like feature allowing users to exchange secret codes — letting WhatsApp users combat what are known as “man in the middle” attacks.

By blocking the heavily encrypted WhatsApp service while making less secure applications like WeChat available to the public, the Chinese government has herded its internet users toward methods of communication that it can reliably monitor.

Residents of mainland China can still use services like WhatsApp if they first connect to virtual private networks that provide them with communications channels to servers outside the Chinese mainland. But the government has also been cracking down on virtual private networks in recent months — and even when those networks appear to be working, they sometimes do not allow access to services that the government is particularly targeting.