WASHINGTON — For the first time, a federal judge has struck down part of the sweeping antiterrorism law known as the 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 U.S. District Court, said in a decision made public 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 is part of a broader set of prohibitions that the Bush administration has relied on heavily in prosecuting people in Lackawana, 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 have stopped writing political material and helping organize peace conferences for fear that they would be prosecuted.

Collins agreed that the ban on providing advice and assistance to terrorists was "impermissibly vague" and blocked the Justice Department from enforcing it against any of 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," 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, Collins sided with the government in rejecting some of the plaintiffs' legal arguments, and she declined to grant a nationwide injunction against the Justice Department.