WASHINGTON — House lawmakers who spent the last 16 months investigating the practices of the world’s largest technology companies said on Tuesday that Amazon, Apple, Facebook and Google had exercised and abused their monopoly power and called for the most sweeping changes to antitrust laws in half a century.

In a 449-page report that was presented by the House Judiciary Committee’s Democratic leadership, lawmakers said the four companies had turned from “scrappy” start-ups into “the kinds of monopolies we last saw in the era of oil barons and railroad tycoons.” The lawmakers said the companies had abused their dominant positions, setting and often dictating prices and rules for commerce, search, advertising, social networking and publishing.

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To amend the inequities, the lawmakers recommended restoring competition by effectively breaking up the companies, emboldening the agencies that police market concentration and throwing up hurdles for the companies to acquire start-ups. They also proposed reforming antitrust laws, in the biggest potential shift since the Hart-Scott-Rodino Act of 1976 created stronger reviews of big mergers.

“Our investigation leaves no doubt that there is a clear and compelling need for Congress and the antitrust enforcement agencies to take action that restores competition, improves innovation and safeguards our democracy”

Jerrold Nadler, Democrat of New York and chairman of the judiciary committee, and David Cicilline, Democrat of Rhode Island and chairman of the antitrust subcommittee, said in a joint statement.

The House report is the most significant government effort to check the world’s largest tech companies since the government sued Microsoft for antitrust violations in the 1990s. It offers lawmakers a deeply researched road map for turning criticism of Silicon Valley’s influence into concrete actions.

“This pattern of behavior raises questions about whether these firms view themselves as above the law, or whether they simply treat lawbreaking as a cost of business.”