WeChat, Weibo and Baidu under investigation
China's largest social media platforms - Weibo, WeChat and Baidu Tieba - are under investigation for alleged violations of cyber security laws.
The Office for Cyberspace Administration said the three platforms had failed to police content on their sites.
It said people had been using the platforms to spread terrorism-related material, rumours and obscenities.
The breaches "jeopardised national security," the administration said.
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.
The authorities in China heavily censor the internet, routinely blocking content or search terms and removing posts they consider sensitive.
They also block 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.
Cyberspace Administration of China has put three most powerful social media sites -- WeChat, Weibo and Baidu Tieba under investigation, as the administration says those platforms failed to police content on their sites and thus breached cyber security laws made in May.
The government body points out that netizens are found to take advantage of those sites to circulate terrorism-related material, rumors and obscentity, thereby jeopardizing national security.
The investigation was followed by the clampdown on popular celebrity gossip sites in June, which were reported to have corrupted "core socialist values".
Besides, this summer has also witnessed a clampdown by the Chinese authorities on so-called Virtual Private Networks, or VPN, through which people can get around the cyber censorship system and browse foreign blocked websites such as Google, Facebook and Twitter, etc.
John Sudworth, the BBC commentator speculates that all of these may be down to the run-up of a key Chinese Communist Party's meeting this autumn.