The encrypted messaging service WhatsApp has suffered intermittent disruptions in China ahead of a Communist Party meeting next month.
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.
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, Xi has promoted the notion of "internet sovereignty," or the right of Beijing and other governments to dictate what their publics 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, didn't 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 over 1.2 billion users.
