Epic Games Inc. alleges that Apple Inc.¡¯s App Store has left users and developers ¡°trapped¡± in an anticompetitive marketplace, while the iPhone maker accuses the creator of Fortnite of a ¡°fundamental assault¡± on a business model that has enriched millions of developers.

The two companies that were once close allies came out swinging Monday at the start of a trial before a federal judge who must decide whether Apple is monopolizing the marketplace for applications that run on its ubiquitous phones.

The trial in Oakland, California, is the first test of a broader backlash Apple is facing ¡ª with billions of dollars in revenue on the line ¡ª from global regulators and some app developers who say its standard App Store fee of 30% and other policies are unfair and self-serving. The fight with Epic blew up in August when the game-maker told customers it would begin offering a discounted direct purchase plan for items in Fortnite, and Apple then removed the game app, cutting off access for more than a billion iPhone and iPad customers.