Instagram has reportedly been blocked in China following a weekend of pro-democracy protests in Hong Kong.
Photos shared on Weibo, a microblogging platform similar to Twitter, also seem to be hidden if they contain certain keywords.
Searches for phrases like "Occupy Central" and "Hong Kong students" are blocked, says the BBC's Beijing bureau.
The Occupy Central movement and students in Hong Kong are leading the protest.
Instagram told the BBC it was aware of the reports and was looking into the situation.
"It's commonplace for China's internet censors to go into overdrive during politically sensitive events," said Celia Hatton, the BBC's correspondent in Beijing.
"However, the decision to block Instagram, a relatively popular photo-sharing application, takes the attempt to manage communications around the Hong Kong protests one step further, revealing Beijing's fears that some in mainland China might be inspired by activists in Hong Kong."
Following a call from student activist leader Joshua Wong, 100,000 people downloaded Bluetooth-powered messenger app Firechat, founding company Open Garden told news website Tech in Asia.
Firechat allows users to take part in group chats with between two and 10,000 people, without the need for an internet connection.
Using a technology known as mesh networking, messages can be sent to people within the immediate vicinity, as long as they too have the app installed. However, discussions are not private, and can be seen by anyone in the area.