In 2014, when thousands of Ukrainians took to the streets to protest the Kremlin-backed government of Viktor Yanukovych, they relied on Zello, an app that allows users to talk one-on-one, like a walkie-talkie, or in broadcast modes that can reach hundreds or thousands of people at once.. Since its founding in 2007, Zello has played a key role in protest and activist movements in Turkey, Hong Kong, Venezuela, and the Arab world. On Wednesday, its roughly 400,000 users in Russia were blocked from using the service, according to Bill Moore, the Austin-based company’s CEO.

“This action follows a notice we received last week from the Russian regulator Roskomnadzor that Zello is not in compliance with a law that governs information distribution brokers,” Moore wrote in a draft blog post provided to Defense One. Zello also responded to the law with this blog post in Russian.

The law in question (Federal Law of 27.07.2006 number 149-FZ) forces companies that provide communication and messaging services over the Internet — including Zello, along with email service providers, social networking services, and the like — to give user data to law enforcement upon request and share all encryption keys with the FSB, the Russian security service.

Moore told Defense One the request was “not serious,” meaning there was no way to comply.

The law has apparently been used to block only one other major service: LinkedIn, in November.

“It’s kind of strange. Why LinkedIn? Why Zello? Twitter wouldn’t comply, neither would Google, or Facebook,” Moore said over the phone.

The block was imposed just weeks after thousands of young people took to the streets in Moscow and across Russia, decrying corruption in the government of Vladimir Putin. More than 500 protesters were detained. The government has promised a crackdown.

Moore said Zello has seen “pretty good growth” in Russia. Its main users in the country are taxi and truck drivers, but the app is also popular with protestors and others.

“Russians use the app to connect with family members and friends and to participate in social, political, and humanitarian conversations and events. But it is also used by search and rescue personnel, taxi services, law enforcement, and drivers needing assistance,” Moore said in a statement.

“We didn’t know if it was a threat or they would really do it. When we announced [the block notice] there was a broad, vocal opposition, including among truck drivers,” he said over the phone.

Moore said Zello was working to restore service via workarounds so that users did not have to rely on a virtual private network to use the service, but thinks that might be more difficult than it was in Venezuela or other countries. Still, at press time, Zello had found a workaround.