A vast mobile messaging network that functions independently of cellular networks has sprung up amid the demonstrators clogging Hong Kong’s financial district. It provides a way for hundreds of thousands of protestors to communicate even when the cellular network is clogged – or if Chinese authorities ultimately decide to shut it down.

Much as Arab Spring protestors flocked to Twitter and Facebook to share information on protests and the actions of authorities (see “Streetbook” and “People Power 2.0”), many protestors in Hong Kong have turned to a messaging app called FireChat. It has phones communicate directly with one another via Wi-Fi or Bluetooth, instead of connecting to a cellular or Wi-Fi network (see “The Latest Chat App for iPhone Needs No Internet Connection”).

When you open the FireChat app you can participate in text chat rooms with people within 100 to 200 feet, even if there’s no connection to cellular or Wi-Fi networks. All chats in such rooms are visible to all people in the room.

The crowds in Hong Kong are protesting the Chinese government’s effort to handpick candidates for 2017 elections for the semi-autonomous region’s chief executive. Citizens had expected to be able to choose from an unrestricted slate of candidates.

So far, the Chinese government has censored news coverage and discussion of the protests in social media – not actually shut down the network. But it has done so in the past; in 2009 the government shut down the network in its Xinjiang province amid protests there.

The rapid spread of FireChat started after a teenager in Hong Kong put out the word Saturday in various social media posts asking people to download the app, says Christophe Daligault, a vice president at Open Garden, the San Francisco-based startup that made the app.

From Saturday to Monday, it was the most popular iPhone app download on both Google Play and in Apple’s iTunes store in Hong Kong, surpassing Twitter, Facebook, or WhatsApp.

During that time, more than 200,000 people in Hong Kong downloaded the app from one of those two stores. Users in Hong Kong sent a total of about two million messages via FireChat on those days, with as many as 33,000 using the app at any one time, says Daligault.

FireChat is already being used for simple forms of organizing, says Daligault. “You could see people creating ‘rooms’ with a specific location—a street corner or landmark—and saying ‘How many masks do we need?’ and ‘Who is going to bring the water?’ and ‘Today it is tear gas, tomorrow it could be water cannon,’ ” he says.

The app has also been used widely in Iraq since an Internet shutdown in June and an increase in Internet filtering by the government there.

However, data sent via FireChat is not encrypted. Indeed, Daligault was only able to see the content of the chats because the phones were still connected to the Internet; the company can see the content of the messages on their servers. This means the Chinese authorities could have done so, too. Right now, FireChat is not a good way to engage in communications more sensitive than planning for water or batteries. Open Garden is currently working to add encryption to the FireChat app.

Hal Roberts, a fellow at Harvard’s Berkman Center for Internet & Society who has done extensive research on Internet censorship and surveillance, says the app would do nothing to stop surveillance. “Best practice for activist security in authoritarian countries is to not only turn off, but remove the batteries from any mobile phones when talking about things that should not be surveilled,” says Roberts.

Authorities can often not only listen in to conversations but also pinpoint the location of devices, says Roberts. In some cases, even if the phone is off, authorities might be able to hack a phone and use the microphone to record conversations taking place nearby.

Ad hoc network chat apps like FireChat could be made more powerful by a technology in development that would let phones link up over distances of up to 500 meters (see “Future Smartphones Won’t Need Cell Towers to Connect”). However, cellular networks will retain some control over how devices use that LTE Direct technology, so it may be possible for authorities to block its use.