NEW YORK -

A federal judge struck down a key part of the USA Patriot Act on Thursday in a ruling that defended the need for judicial oversight of laws and bashed Congress for passing a law that makes possible “far-reaching invasions of liberty.”

U.S. District Judge Victor Marrero immediately stayed the effect of his ruling, allowing the government time to appeal. Justice Department spokesman Dean Boyd said: “We are reviewing the decision and considering our options at this time.”

The ruling handed the American Civil Liberties Union a major victory in its challenge of the post-Sept. 11 law that gave broader investigative powers to law enforcement.

The ACLU had challenged the law on behalf of an Internet service provider, complaining that the law allowed the FBI to demand records without the kind of court supervision required for other government searches. Under the law, investigators can issue so-called national security letters to entities like Internet service providers and phone companies and demand customers’ phone and Internet records.

In his ruling, Marrero said much more was at stake than questions about the national security letters.

He said Congress, in the original USA Patriot Act and less so in a 2005 revision, had essentially tried to legislate how the judiciary must review challenges to the law. If done to other bills, they ultimately could all “be styled to make the validation of the law foolproof.”

Noting that the courthouse where he resides is several blocks from the fallen World Trade Center, the judge said the Constitution was designed so that the dangers of any given moment could never justify discarding fundamental individual liberties.

He said when “the judiciary lowers its guard on the Constitution, it opens the door to far-reaching invasions of liberty.”

Regarding the national security letters, he said, Congress crossed its boundaries so dramatically that to let the law stand might turn an innocent legislative step into “the legislative equivalent of breaking and entering, with an ominous free pass to the hijacking of constitutional values.”

He said the ruling does not mean the FBI must obtain the approval of a court prior to ordering records be turned over, but rather must justify to a court the need for secrecy if the orders will last longer than a reasonable and brief period of time.

A March government report showed that the FBI issued about 8,500 national security letter, or NSL, requests in 2000, the year prior to passage of the USA Patriot Act. By 2003, the number of requests had risen to 39,000 and to 56,000 in 2004 before falling to 47,000 in 2005. The overwhelming majority of the requests sought telephone billing records information, telephone or e-mail subscriber information or electronic communication transactional records.

The judge said that through the NSLs, the government can unmask the identity of Internet users engaged in anonymous speech in online discussions, can obtain an itemized list of all e-mails sent and received by someone and can then seek information on those communicating with the individual.

“It may even be able to discover the web sites an individual has visited and queries submitted to search engines,” the judge said.

Marrero’s lengthy judicial opinion, akin to an eighth-grade civics lesson, described why the framers of the Constitution created three separate but equal branches of government and delegated to the judiciary to say what the law is and to protect the Constitution and the rights it gives citizens.

Marrero said the constitutional barriers against governmental abuse “may eventually collapse, with consequential diminution of the judiciary’s function, and hence potential dire effects to individual freedoms.”

In that event, he said, the judiciary could become “a mere mouthpiece of the legislature.”

Marrero had ruled in 2004, on the initial version of the Patriot Act, that the letters violate the Constitution because they amounted to unreasonable search and seizure. He found free-speech violations in the nondisclosure requirement, which for example, disallowed an Internet service provider from telling customers their records were being turned over to the government.

After he ruled, Congress revised the Patriot Act in 2005, and the 2nd U.S. Circuit Court of Appeals directed that Marrero review the law’s constitutionality a second time.