If they weren’t already doing so, users of WeChat, China’s most popular chat app, may now want to think twice about saying anything critical of the government—even in a private chat.

Chinese authorities last month issued a document titled “Regulation on Collecting and Using Electronic Data as Evidence” (link in Chinese), stating that both private messages and public comments on social media sites can now be used as criminal evidence against defendants in court. The regulation, which went into effect on Saturday (Oct. 1), is a further escalation of the Chinese government’s efforts to police the internet.

This isn’t the first time Chinese authorities have published regulation about collecting online information. Article Seven in the 2005 Electronic Signature Law and the 2015 Civil Procedure Law of China (links in Chinese) both state that messages received or stored on electronic platforms can be presented as evidence in court. But the new regulation specifically names brands like microblogging site Weibo and Moments, a Facebook Newsfeed-esque feature on WeChat, and notes that private online communications like instant messages and group chats remain subject to scrutiny.

Jointly issued by the Supreme People’s Court, the Supreme People’s Procuratorate, and the Ministry of Public Security, the regulation states that courts will collect “electronic data” to investigate legal cases, defining the term as including but not limited to:

Text documents, pictures, videos, electronic certificates, computer programs, and other related electronic documents.

Chinese courts are already increasingly using chat and social media records. From September 2015 to August 2016, for example, the Xicheng District Court in Beijing heard 2,047 cases involving peer-to-peer money lending services, according to court records. Private chat records, obtained mostly from WeChat and QQ (WeChat’s predecessor, also owned by Chinese tech giant Tencent) were used as supporting evidence. Zhao Yan (link in Chinese, registration required), a presiding judge from the Xicheng District Court, even said that online chat records could serve as the key to determining the judgment of a case.

But people are concerned that the new regulation will be used to further stifle free expression online, violate people’s privacy, and suppress voices that are critical of the government. Chinese authorities have already used online discussions as grounds for detention. In March, for example, police detained two ethnic Mongolians after they expressed support on WeChat for fellow Mongolians that had been arrested for taking part in protests.

“It legitimizes the public security bureau’s actions to take away people’s electronic data in the name of investigation,” said Patrick Poon, researcher at Amnesty International in Hong Kong. He added that while the geographic boundaries of the rules are not specified, he believes it likely applies to data on domestic servers, and to people inside China including foreigners. Tencent did not respond to a request for comment from Quartz.

Chinese internet users have reacted with panic. “Oh my, this scares me and I just deleted all the ‘bad’ opinions on my Weibo and WeChat upon reading the regulation,” commented Weibo user Mr. Zhang (link in Chinese, registration required), in response to a post from a state media report about the regulations. Others noted how the rules might hinder freedom of speech. ”We should all speak with caution from now on. Don’t call for freedom of speech, there is no such thing in our country. Common people can’t do anything other than be slaughtered like a fish,” Weibo user Frank Lee wrote (link in Chinese, registration required).