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 — with billions of dollars in revenue on the line — 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.