The chief executive of DuckDuckGo on Thursday described Google as a monopoly that has hurt competition and consumers through its scale and command over the tech industry, in the first testimony of a rival in the federal trial of the Justice Department’s antitrust lawsuit against the search giant.

Gabriel Weinberg of DuckDuckGo said that Google’s deals to make its search engine the default on browsers and other platforms blocked its ability to compete effectively. DuckDuckGo, with only 2.5 percent of the U.S. market for search, tried to negotiate with other companies to make its privacy-focused search engine a default. But it was continually turned down because of Google’s deals with those tech partners, he said.

“We ultimately decided after three years of trying this that it was a quixotic exercise because of the contracts,” Mr. Weinberg said.

The trial of the Justice Department’s antitrust complaint against Google is the government’s first monopoly case in two decades, and it is expected to sweep in many of the biggest tech companies. Executives at Apple, Microsoft and Verizon are also expected to testify in the trial, which could reshape how consumers get information online.

The Justice Department has argued that Google has violated competition laws by maintaining its monopoly through the deals that make it the default access point in users’ search for online information across multiple platforms.

Google has defended its business partnerships, saying the companies, including Apple and Samsung, have chosen to make Google the default on their devices because of Google’s superior quality. Google said consumers can easily adjust settings on their browsers to change search engine defaults to alternatives like DuckDuckGo.

Mr. Weinberg said on Thursday that any switches to default search engines take far more steps than Google says.

“It’s all just way harder than it needs to be,” he said.

Mr. Weinberg didn’t disclose details of business negotiations in public proceedings but continued his testimony in a closed session on day nine of the antitrust trial.

Judge Amit P. Mehta of the U.S. District Court for the District of Columbia has closed many hours of testimonies at the request of Google and other companies that have argued to keep documents and testimony of sensitive business matters private.