WASHINGTON -- A majority of justices on the Supreme Court expressed concerns about a law banning depictions of cruelty to animals, in a spirited debate about the extent of government power to suppress speech.

Two conservative justices, Antonin Scalia and Samuel Alito, staked out opposite positions, raising analogies such as bullfighting and gladiatorial contests to define what might be acceptable.

The new session of the Supreme Court opened with a battle over the First Amendment. WSJ's Jess Bravin describes a case before the high court on whether images of dog fighting are protected under freedom of speech.

Justice Scalia repeatedly criticized the government's effort to liken animal cruelty videos to child pornography -- which, like other forms of obscenity, receives no First Amendment protections under earlier court decisions.

"This is something quite different," he said. Hunting and other practices that wound or kill animals, sometimes painfully, are neither prurient nor, in some places, illegal, Justice Scalia observed.

"What if I am an aficionado of bullfights and I think, contrary to the animal cruelty people, that they ennoble both beast and man," Justice Scalia said. "I would not be able to market videos showing people how exciting a bullfight is."

Deputy Solicitor General Neal Katyal, representing the Obama administration, said that lawmakers, in the law's legislative history, "explicitly exempted Spanish bullfighting" as an example of educational and artistic works not covered by the ban.

The 1999 law was inspired by congressional revulsion at "crush" videos, where barefoot or stiletto-heeled women crush small animals to death. The statute's text criminalizes more than fetish movies, outlawing any visual or sound recording "in which a living animal is intentionally maimed, mutilated, tortured, wounded, or killed," if doing so would be illegal in the place the depiction was made, sold or possessed.

In the only successful prosecution under the law, a Virginia man, Robert Stevens, was convicted for compiling and selling dog-fighting videos.

A federal appeals court in Philadelphia struck down the law, and the Justice Department appealed to the Supreme Court.

The government argues that the law is constitutional because it exempts works with "serious religious, political, scientific, educational, journalistic, historical, or artistic value."

Justice Stephen Breyer questioned whether those exceptions were too vague. "Why not do a simpler thing? Rather than let the public guess as to what these words mean, ask Congress to write a statute that actually aims at those frightful things that it was trying to prohibit," Justice Breyer said.

Robert Stevens, of Pittsville, Va., pictured in 2005, was sentenced in federal court to 37 months for selling dog-fighting videos. Associated Press

In their questions, Justices John Paul Stevens, Ruth Bader Ginsburg and Sonia Sotomayor also seemed troubled by the animal cruelty law, pointing out what they thought were its inconsistencies or vagueness.

Justice Sotomayor cited a documentary on pit bulls that she said had "much, much more footage" of "actual animal cruelty" but wasn't considered unlawful.

"The line will sometimes be difficult to draw," Mr. Katyal said.

Justice Alito, in his questions, suggested the government could suppress depiction of repugnant or barbaric activities.

"Suppose I am an aficionado of the sort of gladiatorial contests that used to take place in ancient Rome...where the gladiators fight to the death," he said. "Do you have any doubt that that could be prohibited?"

What if human sacrifices were legal in a foreign land, Justice Alito said. "People here would probably love to see it. Live, pay per view, you know, on the Human Sacrifice Channel."

Or, he said, "suppose you have the Ethnic Cleansing Channel on cable TV," depicting crimes against humanity overseas. "Congress couldn't prohibit that?"

"Just because we have this really disgusting, despicable channel doesn't mean that we automatically ban it," said Patricia Millett, a lawyer representing Mr. Stevens.

In a friend of the court brief signed by 26 states, law enforcement officials said the federal law had augmented their own efforts to fight animal cruelty, arguing that dog fighting and similar endeavors were often associated with gambling, drug activity and other crimes.

A decision is expected by July.

(U.S. v. Stevens)

Write to Jess Bravin at jess.bravin@wsj.com