WASHINGTON — The Social Security Administration is closing field offices and reducing services to the public even as demand for those services surges with the aging of the baby boom generation, according to a bipartisan Senate committee report.

The report, to be issued Wednesday by the Senate Special Committee on Aging, says the agency has closed more than two dozen field offices in the last year, generally without considering the needs of communities and without consulting beneficiaries or field office managers.

In deciding whether to close field offices, the Social Security Administration “excludes both its own managers and the affected public,” and the decisions often appear arbitrary, the report says.

The committee’s chairman, Senator Bill Nelson, Democrat of Florida, said, “Seniors are not being served well when you arbitrarily close offices and reduce access to services.”

He added, “The closure process is neither fair nor transparent and needs to change.”

The field offices served over 43 million people last year. About 10 percent of the visitors filed for benefits, and 30 percent were seeking new or replacement Social Security cards.

In testimony prepared for a committee hearing on the issue on Wednesday, Nancy A. Berryhill, a deputy commissioner at the agency, said its budget and work force had not kept pace with what she described as “a staggering 27 percent increase” in claims for retirement benefits, to 3.3…