The Bush administration's response to the Sept. 11, 2001, terrorist attacks has dangerously undermined U.S. scientific enterprise and national security by abridging the constitutional and academic freedoms that have long fostered the nation's technical superiority, according to a report released yesterday by the American Civil Liberties Union.

A dramatic increase in the designation of scientific documents as "classified"; the emergence of new, vaguely defined categories of "sensitive but not classified" information; inordinately strict constraints on foreign students -- a mainstay of American science; and restricted access to equipment crucial to the advancement of U.S. research all represent overreactions to the terrorist threat and have left the nation less prepared for future challenges, said the 35-page report, "Science Under Siege."

"Even at a time when fears of terrorism run so high -- especially at such a time -- we must resist the temptation to allow the crude, excessive and questionably effective regime of secrecy that dominates our security agencies to cloud the open operation and steady progress being made under our scientific tradition," the ACLU report concluded.

An administration official denounced the report as inaccurate and its authors as insufficiently cognizant of the threats facing the United States.

"The report has more to do with politics than science and lacks credibility," said Robert Hopkins, a spokesman for the White House Office of Science and Technology Policy. The office is headed by President Bush's science adviser, John H. Marburger III, who was unavailable for comment.

"The administration has worked in good faith with serious members of the scientific community, including the National Academies, to determine the best way to enable the conduct of science without providing terrorists the means to meet their ends," Hopkins said.

The ACLU report echoed concerns expressed by others in recent years over an alleged pattern of sacrificing science for political goals, including the editing of scientific reports at odds with White House policies on global warming, mercury emissions, contraceptives and other topics. A letter expressing alarm about such practices, circulated by the Union of Concerned Scientists, has accumulated signatures from thousands of scientists, including 49 Nobel laureates.

The new report makes few claims not previously made by others but documents some trends with added precision. It notes that the Bush administration not only has reversed a Clinton administration effort to declassify marginally sensitive documents but also has created many vaguely defined categories of quasi-secret information. That has denied experts access to thousands of pages of information about public health, food safety, energy production and other issues that could benefit from open research.

"The creation and widespread use of various terms to label and control so-called 'sensitive' information represents a major extension of the government's power to hide its activities and information," the ACLU report said.

The report also highlights several initiatives aimed at keeping potential terrorists from entering the country but that, the report asserts, have done little more than keep thousands of innocent foreigners, including countless scientists, out of the country.

Over the past 20 years, the report notes, noncitizens have accounted for more than half of the increase in doctorates earned in the United States and more than half of the students enrolled in U.S. science and engineering programs. Yet because of visa delays and rejections related to anti-terrorism efforts, foreign student enrollment fell in 2003 for the first time in three decades -- with most of that drop in the sciences. More than 75 percent of international students and scholars who missed their start dates because of visa delays were in the physical or biological sciences or engineering.

The ACLU report also warns of a new effort by the Commerce Department to restrict the dissemination of fundamental scientific findings by academics, who have long been exempt from such restrictions.

Hopkins offered documentation showing that visa-delay rates have recently dropped, and said the Commerce effort is still open for public comment.

Some of the report's strongest language relates to the way the administration has filled vacancies on scientific advisory committees.

The administration "has repeatedly and blatantly sought to bias the scientific advice produced by such panels by dismissing experts whose opinions are politically inconvenient and replacing them with those whose research and advice appears driven by political ideology and their ties to industry," the report said, noting cases in which candidates were asked whether they had voted for Bush.