European authorities fined Google a record $5.1 billion on Wednesday for abusing its power in the mobile phone market and ordered the company to alter its practices, in one of the most aggressive regulatory actions against American technology giants and one that may force lasting changes to smartphones.

The European Union’s antitrust fine of 4.34 billion euros was almost double the bloc’s fine against Google last year over the company’s unfair favoring of its own services in internet search results. The penalty’s size highlighted Europe’s increasingly bold stance against the power of American tech firms, even as officials in the United States have taken a largely hands-off approach to the companies.

The fine was coupled with remedies that would effectively loosen Google’s grip over its Android software, which is used in 80 percent of the world’s smartphones and is a key part of the Silicon Valley company’s business. Those changes, which European regulators ordered to take effect in 90 days, undercut Google’s ability to automatically include its own search and other apps in mobile devices, opening it to more competition in a market that it has dominated.

“Google has used Android as a vehicle to cement the dominance of its search engine,” said Margrethe Vestager, Europe’s antitrust chief. “These practices have denied rivals the chance to innovate and compete on the merits. They have denied European consumers the benefits of effective competition in the important mobile sphere.”

The size of the fine, she added, “reflects the seriousness and the sustained nature” of Google’s actions.

Google said it would appeal the decision, and the case is very likely to drag on for years. The company must deposit the fine in a holding account while the legal process unfolds. If Google ultimately loses an appeal, the money will be distributed among the European Union’s member states.

Regardless of an appeal, if Google does not start altering its mobile phone practices in 90 days, it faces penalties of up to 5 percent of the worldwide average daily revenue of its parent company, Alphabet.

Sundar Pichai, Google’s chief executive, said on Twitter that “rapid innovation, wide choice, and falling prices are classic hallmarks of robust competition.”

“Android has enabled this and created more choice for everyone, not less,” he added.

The long-anticipated ruling arrived at a politically delicate period, with Europe and the United States engaged in an escalating trade conflict in which both sides have imposed tariffs on an array of products, from alcohol to aluminum. Last week, on a trip to Brussels, President Trump reiterated his complaints that American businesses were at a disadvantage in Europe. Jean-Claude Juncker, president of the European Commission, the bloc’s executive arm, is to visit Washington next week for talks with Mr. Trump.