The encrypted messaging service WhatsApp suffered intermittent disruptions in China on Tuesday as communist authorities tightened censorship ahead of a major ruling party meeting.
Attempts to set up new WhatsApp accounts on some mobile phones were met with network error messages.
Others reported difficulty sending images and video on the service, which is owned by Facebook and offers more privacy than government-monitored Chinese social media.
Chinese authorities are tightening controls on social media ahead of the party congress next month at which President Xi Jinping is due to be appointed to a second five-year term as leader.
The ruling party encourages internet use for education and business but tries to block access to material that calls for political change or is deemed to encourage protests.
Since coming to power in 2012, Mr Xi has promoted the notion of "internet sovereignty," or the right of Beijing and other governments to dictate what the public can do and see online.
Private sector operators of Chinese social media are required to delete banned material.
China has the world's biggest population of internet users, with some 730 million people online.
The agency that enforces online censorship, the Cyberspace Administration of China, did not respond to questions sent by fax about whether Chinese regulators were causing the WhatsApp disruption.
Facebook declined to comment.
WhatsApp is one of the world's most popular messaging services, with more than 1.2 billion users.
On Monday, the internet agency announced it was fining the parent of the popular Chinese messaging service WeChat, Tencent Ltd, and two other social media services, for failing to fully enforce censorship.
The agency said WeChat, Sina Corp's microblog service Sina Weibo and search giant Baidu Inc's Tieba allowed users to post false rumours and material that was pornographic or threatened national security.
The agency gave no figure for a penalty but cited a law under which violators can be fined up to 500,000 yuan (€63,000) or closed in the event they fail to correct a violation.
Also this month, regulators announced that organisers of messaging groups would be held responsible for managing their content.
Meanwhile, Beijing is cracking down on the use of virtual private networks, technology that creates encrypted links between computers and can be used to see blocked websites abroad.
Regulators say only government-approved VPNs are allowed and how they can be used will be restricted, potentially cutting off millions of Chinese entrepreneurs and other individual users.
Users of WhatsApp in China reported similar disruptions in July, prompting fears authorities were blocking access.
Chinese filters block access to other global social media including Twitter, Facebook and YouTube without a VPN.
Officials argue that such services operating beyond their control pose a threat to national security.
Chinese authorities increasingly pay attention to encrypted messaging apps.
Users in China switched to WhatsApp after authorities blocked access to another encrypted service, Telegraph, following a 2015 crackdown on lawyers and human rights activists.