WASHINGTON — The National Security Agency vacuumed up more than 151 million records about Americans’ phone calls last year via a new system that Congress created to end the agency’s once-secret program that collected domestic calling records in bulk, a report disclosed Tuesday.

Although the number is large on its face, it nonetheless represents a massive reduction from the amount of information the agency gathered previously. Under the old system, it collected potentially “billions of records per day,” according to a 2014 study.

The new report, an annual surveillance review published by the Office of the Director of National Intelligence, offered the first glimpse of how the new system is working. That the National Security Agency still collected such a large volume of calling data, even if it was only a fraction of what the agency once gathered, showed the challenge of conducting 21st-century surveillance and data monitoring within constraints set up to protect Americans’ privacy.

Since the Sept. 11, 2001, attacks, the agency has analyzed large amounts of communications metadata — records showing who contacted whom, but not what they said — to hunt for associates of terrorism suspects. For years, it did so by collecting domestic call records in bulk.