WASHINGTON — In a major and muscular First Amendment ruling, the Supreme Court on Tuesday struck down a federal law that made it a crime to create or sell dogfight videos and other depictions of animal cruelty.

Chief Justice John G. Roberts Jr., writing for the majority in the 8-to-1 decision, said the law created “a criminal prohibition of alarming breadth” and that the government’s aggressive defense of the law was “startling and dangerous.”

The decision left open the possibility that Congress could enact a narrower law that would pass constitutional muster. But the existing law, Chief Justice Roberts wrote, covered too much speech that depicted lawful activities.