ASHINGTON, Sept. 17 — Under pressure from lawmakers and civil rights groups over the Justice Department's antiterrorism initiatives, Attorney General John Ashcroft reversed course today and agreed to declassify data showing how often federal agents had demanded records from libraries and other institutions.

Civil rights advocates said the decision, which Mr. Ashcroft disclosed today in a telephone call to the president of the nation's biggest library association, was a significant victory in their efforts to lift the secrecy surrounding the antiterrorism legislation known as the USA Patriot Act.

Department officials said that in the next several days they would make public data showing how often agents had examined library records, business documents, computer data and other material in investigations. Section 215 of the antiterrorism act , which gave the department expanded authority to demand such records, has drawn strong objections from many librarians, civil rights advocates and community groups who say it gives the government too much power to spy on private activities of Americans.

Mr. Ashcroft says the Justice Department has used its powers sparingly. But until now he has refused requests from members of Congress and others to provide details about the department's use of the section of the law dealing with library records and other documents, saying the information is classified.

But Mr. Ashcroft told Carla Hayden, president of the American Library Association, in a telephone call today that he would relent and release the information.

Ms. Hayden and several lawmakers said the decision appeared to be a response to criticism that Mr. Ashcroft received this week after suggesting in a speech that the library association and other critics had helped fuel "hysteria" and misunderstandings over the government's use of the Patriot Act in libraries.

Emily Sheketoff, executive director of the association's Washington office, said, "I think the Justice Department was taken by surprise by the negative reaction that his attack on librarians had."

Mark Corallo, a department spokesman, said it was uncertain what form the public release of the documents would take. But, he said, "the numbers that everyone is so concerned about will be released." He said Mr. Ashcroft had "made light of all the criticism, but he wanted to make sure the public understands what we're actually doing."

"He felt it was in the public interest and the national security interest to have these numbers declassified," Mr. Corallo said.