The Justice Department and 38 states and territories on Tuesday laid out how Google had systematically wielded its power in online search to cow competitors, as the internet giant fiercely parried back, in the opening of the most consequential trial over tech power in the modern internet era.

In a packed courtroom at the E. Barrett Prettyman U.S. Courthouse in Washington, the Justice Department and states painted a picture of how Google had used its deep pockets and dominant position, paying $10 billion a year to Apple and others to be the default search provider on smartphones. Google viewed those agreements as a “powerful strategic weapon” to cut out rivals and entrench its search engine, the government said.

“This feedback loop, this wheel, has been turning for more than 12 years,” said Kenneth Dintzer, the Justice Department’s lead courtroom lawyer. “And it always turns to Google’s advantage.”

Google denied that it had illegally used agreements to exclude its search competitors and said it had simply provided a superior product, adding that people can easily switch which search engine they use. The company also said that internet search extends more broadly than its general search engine and pointed to the many ways that people now find information online, such as Amazon for shopping, TikTok for entertainment and Expedia for travel.

“Users today have more search options and more ways to access information online than ever before,” said John E. Schmidtlein, the lawyer who opened for Google.

The back-and-forth came in the federal government’s first monopoly trial since it tried to break up Microsoft more than two decades ago. This case — U.S. et al. v. Google — is set to have profound implications not only for the internet behemoth but for a generation of other large tech companies that have come to influence how people shop, communicate, entertain themselves and work.

Over the next 10 weeks, the government and Google will present arguments and question dozens of witnesses, digging into how the company came to power and whether it broke the law to maintain and magnify its dominance. The final ruling, by Judge Amit P. Mehta of the U.S. District Court of the District of Columbia, could shift the balance of power in the tech industry, which is embroiled in a race over artificial intelligence that could transform and disrupt people’s lives.

A government victory could set limits on Google and change its business practices, sending a humbling message to the other tech giants. If Google wins, it could act as a referendum on increasingly aggressive government regulators, raise questions about the efficacy of century-old antitrust laws and further embolden Silicon Valley.

“It is a test of whether our current antitrust laws — the Sherman Act, written in 1890 — can adapt to markets that are susceptible to monopolization in the 21st century,” said Bill Baer, a former top antitrust official at the Justice Department, adding that Google was “indisputably powerful.”

The case is part of a sweeping effort by the Biden administration and states to rein in the biggest tech companies. The Justice Department has filed a second lawsuit against Google over its advertising technology, which could go to trial as early as next year. The Federal Trade Commission is separately moving toward a trial in an antitrust lawsuit against Meta. Investigations remain open in efforts that could lead to antitrust lawsuits against Amazon and Apple.

The Justice Department filed the case accusing Google of illegally maintaining its dominance in search in October 2020. Months later, a group of attorneys general from 35 states, Puerto Rico, Guam and the District of Columbia filed their own lawsuit arguing that Google had abused its monopoly over search. Judge Mehta is considering both lawsuits during the trial.

The case centers on the agreements that Google reached with browser developers, smartphone manufacturers and wireless carriers to use Google as the default search engine on their products. Since the lawsuit was filed, more than five million documents and depositions of more than 150 witnesses have been submitted to the court. Last month, Judge Mehta narrowed the scope of the trial, while allowing the core claims of monopoly abuse in search to remain.

The trial unfolded on Tuesday in Courtroom 10 at Washington’s federal courthouse, a complex minutes from Capitol Hill. It drew a large crowd, with some people standing in line to enter as early as 4:30 a.m. Officials from the Google rivals Yelp and Microsoft also attended, as did dozens of attorneys and staff from the Justice Department, states and Google after years of work on the case.

Judge Mehta began the proceedings punctually. In the government’s opening statement, Mr. Dintzer focused on the search agreements Google had struck with Apple and others. He referenced internal company documents that described how Google would not share revenue with Apple without “default placement” on its devices and how it worked to ensure that Apple couldn’t redirect searches to its Siri assistant.

“Your honor, this is a monopolist flexing,” Mr. Dintzer said.

In blunt language, Mr. Dintzer also argued that Google had tried to hide documents from antitrust enforcers by including lawyers on conversations and marking them as subject to attorney-client privilege. He showed a message from Sundar Pichai, Google’s chief executive, asking for the chat history to be turned off in one conversation.

“They turned history off, your honor, so they could rewrite it here in this courtroom,” Mr. Dintzer said.

William Cavanaugh, a lawyer for the states, echoed Mr. Dintzer’s concerns about Google’s agreements to become the default search engines on smartphones. He added that Google had limited a product used to place ads on other search engines to hurt Microsoft, which makes the Bing search engine.

In response, Mr. Schmidtlein, Google’s lawyer, argued that the company’s default agreements with browser makers don’t lock up the market the way that the Justice Department said. Browser makers such as Apple and Mozilla both promote other search engines, he said, and it was easy for users to switch their default search engine.

Using a slide show, Mr. Schmidtlein demonstrated the number of taps or clicks required to change the default on popular smartphones. People who wished to switch their search engine but did not know how could search Google for instructions or watch a video tutorial on YouTube, which Google owns, he said.

The government’s evidence was coming from “snippets and out-of-context” emails, he said.

The lawyers also sparred over whether Google was as dominant as the government claimed. The Justice Department and the states said Google competes primarily with broad search engines that act as a single place to look for multiple types of information. But Mr. Schmidtlein said Google’s universe of competitors was wider, including online retailers like Amazon, food delivery apps like DoorDash and travel booking sites like Expedia.

In the afternoon, the Justice Department called Hal Varian, Google’s chief economist, as its first witness to establish that the company had long been aware of its power in search and deliberately tried to sidestep antitrust scrutiny.

In more than three hours of testimony, Mr. Varian was asked about views that he shared with other Google employees on the power of defaults, the threat of Microsoft’s entry into search and his awareness of language that could invite the attention of antitrust regulators. The Justice Department drew from Mr. Varian’s emails and memos from as far back as the early 2000s.

Mr. Varian is scheduled to return to the witness stand on Wednesday.

Nico Grant and Steve Lohr contributed reporting.