"Our data has been effectively hijacked by the U.S. under cover of secret agreements and entirely undisclosed terms," said the civil liberties advocate, Simon Davies, the director of Privacy International, a London-based organization focused on the intrusion on privacy by governments and businesses. "There will be a snapping point, and this may be it."

Initial reaction from global banks was muted, with one executive saying that while the privacy of information was a contentious issue within the industry, the Swift operation had so far generated few complaints.

In Washington on Friday, privacy groups and civil liberties advocates were critical of the program, as were some Democrats and one prominent Republican on Capitol Hill.

The executive director of the American Civil Liberties Union, Anthony D. Romero, condemned the program, calling it "another example of the Bush administration's abuse of power."

Lauren Weinstein, the head of the California-based Privacy Forum, an online discussion group, raised concerns about lack of independent review of the operation. "Oversight is the difference between something being reasonable and something being abuse," he said.

Senator Arlen Specter, Republican of Pennsylvania and chairman of the Senate Judiciary Committee, said he had sent letters on Friday to both Treasury Secretary John W. Snow and Attorney General Alberto R. Gonzales on the issue. While he declined to release the letters, he said he was concerned about the legal authority for the operation.

Image Treasury Secretary John W. Snow, left, with Under Secretary Stuart Levey Friday. Credit... Stephen Crowley/The New York Times

Mr. Specter has been at odds with the administration over another previously secret counterterrorism operation, the National Security Agency's domestic eavesdropping program. The senator said he was particularly troubled that the administration had expanded its Congressional briefings on the financial tracking program in recent weeks after having learned that The New York Times was making inquiries.