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.