Apple’s and Google’s spats with app developers over their cut of revenues exploded into a high-stakes clash on Thursday when the tech giants kicked the wildly popular game Fortnite out of their app stores and the game’s maker hit back with lawsuits. The fight began on Thursday morning with a clear provocation. Epic Games, the maker of Fortnite, started encouraging Fortnite’s mobile-app users to pay it directly, rather than through Apple or Google. Hours later, Apple responded, removing the Fortnite app from its App Store.

Within an hour, Epic opened a multifront war against Apple that appeared months in the making. First, it sued Apple in federal court, accusing the company of violating antitrust laws by forcing developers to use its payment systems. Apple has said that all app developers are subject to the same rules and that its commission is fair. Apple has argued that it spends billions of dollars on the App Store and iPhone technology, creating business opportunities for companies like Epic.

Suing Apple, in particular, serves two goals for Epic: winning in legal court and winning in the court of public opinion, said Rebecca Haw Allensworth, a professor of antitrust at Vanderbilt Law School. Epic is more likely to succeed in the latter, she said. “There is growing business pressure against Apple,” she said, noting an antitrust case would be more complicated and difficult to win.

Mr. Sweeney said that he felt “obligated” to “make this industry a better and fairer place.” “It’s critical to the future of humanity,” he said. “Otherwise you have these corporations who control all commerce and all speech.”