The Reddit blackout has begun. Over 6,000 communities have blacked out and restricted user access in to response to advertising changes. Beginning Monday morning, the developer billing structure has changed, charging thousands of dollars for access to an application programming interface, or API, per CNBC.

The blackout post was captioned: “Don’t Let Reddit Kill 3rd Party Apps!” Alongside the post, the organizers encouraged users to share their complaints with Reddit and to boycott the app. There was also a list of demands, with the most important being the resolution of API issues.

The API change concerns the organizers. Reddit earns revenue from its advertising, but they banned third-party apps from running their own ads or benefiting from Reddit’s advertising platform. This, alongside the proposed API pricing, is a risk for third party developers. This change can end the use of these highly popular apps.

What these third party apps are, are other ways to post and interact with Reddit. Some of them allow users to alter the appearance of their own account, for example. Third party app Apollo would be the one most affected by the billing change. The developer of Apollo, Christian Selig, said the change would cost them $20 million a year, and that would be the end of their app.

Some of the Reddit communities, like humor, gaming, and gadgets, have joined the blackout. The Reddit blackout limits use of interactions on the subreddits—in other words, they’re forcing a boycott. The moderators of these thousands of subreddits have restricted access or privatized them, with the goal of gaining strength in numbers.