China says it is investigating its largest social media platforms - Weibo, WeChat and Baidu Tieba - for alleged violations of cyber security laws.
The Cyberspace Administration said people had been using the three platforms to spread terror-related material, rumours and obscenities.
The breaches "jeopardised national security," the administration said.
China's authorities heavily censor the internet, routinely blocking content or search terms they consider sensitive.
Weibo is a Twitter-like microblogging site, WeChat is an instant messaging mobile app and Baidu Tieba is a popular discussion forum. They are all reported to have hundreds of millions of active users.
China's Cyberspace Administration accused internet users of "spreading violence, terror, false rumours, pornography and other hazards to national security, public safety, social order" on the three platforms.
Baidu expressed "regret" and said it would "actively co-operate with government departments" and "increase the intensity of auditing", Reuters reported.
There was no immediate comment from Tencent, which owns WeChat, or Weibo.
Analysis: China continues to tightens online rules
John Sudworth, BBC News, Beijing
Weibo, WeChat and Baidu's Tieba are among the most powerful social media platforms in the world, each attracting hundreds of millions of users in China.
In China, posts are easily traceable through registered phone numbers and most people already know well the topics and opinions to steer clear of.
But despite the tight surveillance and censorship, dissent still bubbles away and, ahead of a highly sensitive Communist Party Congress this autumn, the authorities are tightening those controls further.
The move to place the three platforms under investigation will almost certainly prompt the sites' owners to do even more to police their own content.
Last month 60 popular celebrity gossip sites were closed overnight for corrupting "core socialist values," and a new regulation released in May requires all online news portals to be managed by Communist Party-sanctioned editorial staff.
In 2016, watchdog Freedom House ranked China as "the world's worst abuser of internet freedom", adding that "harsh punishments for expression and a deteriorating legal environment are significantly undermining civil society activism on the internet".
China blocks foreign social media sites and apps, including Facebook, Instagram and Twitter.
Search engines like Google are blocked, and access to many foreign media outlets is restricted too.
It is also clamping down on users who try to get around the restrictions, by tightening regulations on Virtual Private Networks.