(CNN) -- A federal judge on Thursday ruled that the U.S. government's domestic eavesdropping program is unconstitutional and ordered it ended immediately.
Attorney General Alberto Gonzales said the Bush Administration disagrees with the ruling and has appealed.
"We also believe very strongly that the program is lawful," he said in Washington, adding that the program is "reviewed periodically" by lawyers to determine its effectiveness and ensure lawfulness. (Watch Gonzales defend the constitutionality of the domestic spying program -- 1:50)
The administration secretly instituted the program after the September 11, 2001, attacks on New York and Washington. It gives the National Security Administration authorization to secretly conduct wiretaps without a court order.
In a statement from the White House, Press Secretary Tony Snow said, "The program is carefully administered and targets only international phone calls coming into or out of the United States where one of the parties on the call is a suspected al Qaeda or affiliated terrorist.
"The whole point is to detect and prevent terrorist attacks before they can be carried out," the statement said.
In a 44-page memorandum and order, U.S. District Judge Anna Diggs Taylor struck down the NSA program, which she said violates the rights to free speech and privacy. (Read the complete ruling -- PDF)
The defendants "are permanently enjoined from directly or indirectly utilizing the Terrorist Surveillance Program in any way, including, but not limited to, conducting warrantless wiretaps of telephone and Internet communications, in contravention of the Foreign Intelligence Surveillance Act and Title III," she wrote.
She declared that the program "violates the separation of powers doctrine, the Administrative Procedures Act, the First and Fourth amendments to the United States Constitution, the FISA and Title III."
Her ruling went on to say that "the president of the United States ... has undisputedly violated the Fourth in failing to procure judicial orders."
The lawsuit, filed January 17 by civil rights organizations, lawyers, journalists and educators, "challenges the constitutionality of a secret government program to intercept vast quantities of the international telephone and Internet communications of innocent Americans without court approval."
The complaint was filed in U.S. District Court for the Eastern District of Michigan, based in Detroit. Plaintiffs included branches of the American Civil Liberties Union, the National Association of Criminal Defense Lawyers, the Washington and Detroit branches of the Council on American-Islamic Relations and Greenpeace.
The judge in the case, Taylor, 75, was appointed by President Carter and has been on the Eastern District of Michigan bench since 1979. She is one of the first African-American women to sit on a federal court.
Program under scrutiny
Electronic surveillance programs run by the NSA have been under fire since December, when The New York Times disclosed that the government was listening in -- without obtaining a court order -- on international phone calls involving people suspected of having ties to terrorists.
Some legal scholars said the program is an illegal and unwarranted intrusion on Americans' privacy. The Bush administration defended it as a necessary tool in the battle against terrorism.
Opinion polls suggest the U.S. public has been divided on the NSA program. A CNN poll conducted by Opinion Research Corp. on May 16-17 found that 50 percent of the respondents believe the program was "wrong," while 44 percent believe it was "right." The poll's margin of error was plus or minus 4.5 percent.
The plaintiffs alleged their communications with parties outside the country were being monitored by the NSA's wiretapping program. The complaint said the NSA's surveillance disrupts "the ability of the plaintiffs to talk with sources, locate witnesses, conduct scholarship and engage in advocacy."
On May 26, instead of responding to arguments attacking the legality of the NSA's eavesdropping program, the government filed for dismissal of the case. It cited the "U.S. military and state secrets privilege" and argued the government would not be able to defend the domestic spying program without disclosing classified information.
ACLU official calls ruling 'landmark victory'
"Today's ruling is a landmark victory against the abuse of power that has become the hallmark of the Bush administration," said Anthony D. Romero, the ACLU's executive director.
"Government spying on innocent Americans without any kind of warrant and without congressional approval runs counter to the very foundations of our democracy. We hope that Congress follows the lead of the court and demands that the president adhere to the rule of law."
Senate Minority Leader Harry Reid, a Nevada Democrat, said the ruling necessitated the need for the Bush administration "to work with the Congress to develop effective tools to defeat terrorists."
Senate Majority Leader Bill Frist, R-Tennessee, said he backs the government's appeal of the ruling.
"Terrorists are the real threat to our constitutional and democratic freedoms, not the law enforcement and intelligence tools used to keep America safe," Frist said in a statement.
"We need to strengthen, not weaken, our ability to foil terrorist plots before they can do us harm. I encourage swift appeal by the government and quick reversal of this unfortunate decision."
Republican Congressman Pete Hoekstra, chairman of the House Permanent Select Committee on Intelligence, pointed to the alleged airplane plot that was disrupted last week. "It is disappointing that a judge would take it upon herself to disarm America during a time of war," he said in a statement.
In July, Sen. Arlen Specter, chairman of the Senate Judiciary Committee, said the White House had agreed to submit the program to the FISA court for review.
Specter, R-Pennsylvania, and the White House said they would support legislation that would consolidate about 30 lawsuits filed against the government, transferring them to the FISA court so there would be a single forum in which to litigate them.
But the legislation has not passed through the Judiciary Committee, let alone the Senate, and it may never be approved, as Democrats have raised objections to a number of its key components.
Specter was in India and not immediately available for comment on the ruling.