WASHINGTON — The FBI engaged in widespread and serious misuse of its authority in illegally gathering telephone, e-mail, and financial records of Americans and foreigners while hunting suspected terrorists, the Justice Department’s chief inspector said yesterday.

The FBI’s failure to establish sufficient controls or oversight for collecting the information through so-called national security letters constituted “serious and unacceptable” failures, said Glenn Fine, the internal watchdog who exposed the data-gathering abuses in a 130-page report last week.

Testifying before the House Judiciary Committee, Mr. Fine said he did not believe the problems were intentional, but were generally the result of confusion and carelessness. “It really was unacceptable and inexcusable what happened here,” Mr. Fine said under questioning.

Republicans and Democrats alike sternly warned the FBI that it risks losing its broad surveillance powers if the abuses persist. The threats were the latest blow to the embattled Justice Department and Attorney General Alberto Gonzales, who is already on the defensive and fighting to keep his job over the firings of federal prosecutors.

The White House pushed back yesterday against Democrats demanding answers on the firings of federal prosecutors, refusing to allow President Bush’s top aides to testify publicly and under oath about their roles in the dismissals.

The Senate, meanwhile, voted to strip Mr. Gonzales of his authority to fill U.S. attorney vacancies without Senate confirmation. Democrats contend the Justice Department and White House purged eight federal prosecutors, some of whom were leading political corruption investigations, after a change in the Patriot Act gave Mr. Gonzales the new authority.

White House Counsel Fred Fielding told lawmakers they could interview presidential counselor Karl Rove, former White House Counsel Harriet Miers, and their deputies — but only on the president’s terms: in private, “without the need for an oath,” and without a transcript. Mr. Bush warned Democrats to accept this offer or risk a constitutional showdown from which he would not back down. “We will not go along with a partisan fishing expedition aimed at honorable public servants,” he said.

Democrats’ response to his proposal was swift and firm: They said they would start authorizing subpoenas as soon as tomorrow for the White House aides.

Meanwhile, Democrats said Mr. Fine’s findings related to FBI surveillance abuses were an example of how the Justice Department has used broad counterterrorism authorities Congress granted in the wake of the September 11, 2001, attacks to trample on privacy rights.

In a review of headquarters files and a sampling of just four of the FBI’s 56 field offices, Mr. Fine found 48 violations of law or presidential directives during 2003–2005 and estimated that “a significant number of … violations throughout the FBI have not been identified or reported.”

The bureau has launched an audit of all 56 field offices to determine the extent of the problem.