BEIJING - Chinese censors and opponents of the protests sweeping Hong Kong are engaging in a cat-and-mouse game with demonstrators and commentators in a bid to stop news of the unrest spreading online and, in particular, reaching the mainland.
Tens of thousands of pro-democracy protesters have braved police tear gas to make their views known.
Spreading the word over Western and Chinese social networks is a lot safer, but it is becoming increasingly difficult as mobile phone networks are disrupted and concerns about possible surveillance grow.
The intervention is beyond what is normal for the usually free-talking Hong Kong, even as people are used to Chinese censors scrubbing the Internet in the mainland when mass demonstrations erupt.
On Sunday, users reported that Facebook Inc's photo sharing app Instagram was inaccessible on China's mainland.
Chinese websites, including Baidu Inc's search engine and the Twitter-like Weibo Corp microblog, have set about deleting references to the Hong Kong demonstrations.
Others have reported messages on Tencent Holdings Ltd's hugely popular WeChat messaging app being removed.
"I think it is still quite safe except WeChat, which is China," said Oscar, a 21-year-old student at Hong Kong's Polytechnic University, who uses Facebook and WhatsApp to communicate and plan with other protesters.
"It depends on your phone, because some China (brand) phones, they can detect your messages," he said.
Others in Hong Kong were still using WeChat, but had noted signs of censorship.
"WeChat is not blocked, I think some stuff is being deleted," said Jennie, who, after growing up in mainland China and being educated in the United States, now runs a Hong Kong-based charity.
"I forwarded an article (on Hong Kong) on today and it was deleted. The mainland should think it's good people are expressing ideas on behalf of the mainland government, but they even deleted that. Basically they're preventing the opportunity for dialog, which if you think about it is quite scary."
OFF THE GRID
One of the most popular tools for getting around network disruptions during recent demonstrations is FireChat, an "off-the-grid" messaging app where users can communicate with other users via Bluetooth without being connected to the internet.
It was the most downloaded app from Hong Kong's iOS App Store on Monday, and the more people who use the app, the further its signal is broadcast. The range from one device to the next is 40-70 meters.
Christophe Daligault, chief marketing officer at Open Garden Inc, the San Francisco-based company behind FireChat, said that despite widespread adoption of the app during internet blackouts in Iran and Iraq earlier this year, Hong Kong's embrace of FireChat was by far the largest.
"What we're seeing right now is beyond anything we've seen," he said.
Daligault warned, however, that messages on the app are public and people might want to avoid using their real names. Nevertheless, in a city of 7 million people, FireChat had more than 100,000 new users sign up in Hong Kong in under 24 hours.
As wireless networks went down in parts of Hong Kong on Sunday night, a possible result of saturation or network overload, demonstrators jumped onto FireChat to send updates about the protest and the latest police movements.
Others in Hong Kong posted messages on social networks about an invitation to download an app which some suspected was a virus being sent by the movement's opponents.
Hong Kong's internet is not subject to censorship like websites and apps on the mainland. There, more than 150 Weibo posts in every 10,000 were being censored on Sunday, a record high for 2014, according to censorship watchdog Weiboscope.
Despite that, images of the police reaction to largely peaceful protests were still being shared and discussed on the microblog as of Monday.
"All my friends ... know what's happening in Hong Kong," said Jennie in Hong Kong. "They're tweeting from Weibo and WeChat and forwarding articles. Not expressing personal views, but there are articles being forwarded."
At the same time, Chinese authorities ordered all websites to "immediately clear away information about Hong Kong students violently assaulting the government and about 'Occupy Central'," according to China Digital Times, another censorship watchdog.
"My dad saw an article discussion I forwarded on the 'deeper' issues causing the current situation, and he replied: 'Oh it's been deleted'," said Jennie. "And that's it, he didn't seem to be bothered by it." (Additional reporting by Farah Master in HONG KONG, Jeremy Wagstaff in SINGAPORE and Beijing Newsroom; Editing by Mike Collett-White)