ASHINGTON, Jan. 26 — For the first time, a federal judge has struck down part of the sweeping antiterrorism law known as the USA Patriot Act, joining other courts that have challenged integral parts of the Bush administration's campaign against terrorism.

In Los Angeles, the judge, Audrey B. Collins of Federal District Court, said in a decision made public on Monday that a provision in the law banning certain types of support for terrorist groups was so vague that it risked running afoul of the First Amendment.

Civil liberties advocates hailed the decision as an important victory in efforts to rein in what they regard as legal abuses in the government's antiterrorism initiatives. The Justice Department defended the law as a crucial tool in the fight against terrorists and promised to review the Los Angeles ruling.

At issue was a provision in the act, passed by Congress after the attacks of Sept. 11, 2001, that expanded previous antiterrorism law to prohibit anyone from providing "expert advice or assistance" to known terrorist groups. The measure was part of a broader set of prohibitions that the administration has relied heavily on in prosecuting people in Lackawanna, N.Y., Portland, Ore., Detroit and elsewhere accused of providing money, training, Internet services and other "material support" to terrorist groups.

In Los Angeles, several humanitarian groups that work with Kurdish refugees in Turkey and Tamil residents of Sri Lanka had sued the government, arguing in a lawsuit that the antiterrorism act was so ill defined that they had stopped writing political material and helping organize peace conferences for fear that they would be prosecuted.

Judge Collins agreed that the ban on providing advice and assistance to terrorists was "impermissibly vague" and blocked the Justice Department from enforcing it against the plaintiffs.

"The USA Patriot Act places no limitation on the type of expert advice and assistance which is prohibited, and instead bans the provision of all expert advice and assistance regardless of its nature," Judge Collins wrote in a ruling issued late Friday.

As a result, the law could be construed to include "unequivocally pure speech and advocacy protected by the First Amendment," wrote the judge, who was appointed to the bench by President Bill Clinton.

At the same time, however, Judge Collins sided with the government in rejecting some of the plaintiffs' arguments, and she declined to grant a nationwide injunction against the Justice Department.

Even so, lawyers for the humanitarian groups said they were heartened by the ruling. It came seven weeks after many of the same plaintiffs won a ruling in a separate but related case before a federal appeals court in San Francisco. That court, the United States Court of Appeals for the Ninth Circuit, found that a 1996 antiterrorism law prohibiting anyone from providing training or personnel for terrorist groups was too vague to pass constitutional muster. In recent months, other courts have also challenged the administration's designation of enemy combatants and other aspects of the campaign against terrorism, but the Los Angeles decision was the first by a federal judge to strike down any portion of the Patriot Act.

"The critical thing here is that this is the first demonstration that courts will not allow Congress in the name of fighting terrorism to ignore our constitutional rights," said Nancy Chang, a senior lawyer with the Center for Constitutional Rights, the New York-based organization that brought the lawsuit against the Justice Department on behalf of the humanitarian groups. "By using a broad and vague definition of terrorism, that has a chilling effect on free speech."

The Justice Department, which already sought a review of the related decision in San Francisco, also plans to review Judge Collins's ruling to decide whether it should be appealed, officials said.

Administration officials have made clear that they consider the Patriot Act to be an integral part of their efforts to identify, track and disrupt terrorist activities.

Indeed, President Bush in his State of the Union message last week urged Congress to renew parts of the act that are scheduled to expire in 2005. But the administration may face a tough sell in Congress, with a growing number of lawmakers from both parties questioning whether the government's expanded powers in dozens of areas of law enforcement have infringed on civil liberties. In largely symbolic votes, more than 230 communities nationwide have raised formal objections to the law.

Mark Corallo, a spokesman for the Justice Department, said in a statement on Monday that the language banning expert advice or assistance to terrorists represented only "a modest enhancement" of previous law.

"By targeting those who provide material support by providing expert advice or assistance," Mr. Corallo said, "the law made clear that Americans are threatened as much by the person who teaches a terrorist to build a bomb as by the one who pushes the button."