Most of the affected users said they could not send or receive photos using the chat app, which is owned by Facebook, without a virtual private network.
VPNs are used to skirt Beijing's censorship system – which blocks websites with information that could be critical of the Communist Party such as YouTube, Twitter and foreign news sites – by rerouting internet traffic elsewhere.
Some users said they could not get WhatsApp to connect to the internet on Tuesday without a VPN.
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In a test conducted by the South China Morning Post in the afternoon, two users registered with mainland Chinese mobile numbers were unable to send videos or pictures to each other via WhatsApp.
One of the users then tried, and failed, to send the video and photo files to an overseas number. When that person tried to send a photo to the mainland Chinese user from overseas, the message was received but the photo was only displayed as a loading thumbnail.
There was no problem sending and receiving text messages, and all services appeared normal when connected to a VPN.
Beijing's ongoing campaign to "clean up" the internet has seen VPN providers shut down, celebrity gossip accounts closed and video content restricted.