The Trump administration laid off thousands of federal health workers on Tuesday in a purge that included senior leaders and top scientists charged with regulating food and drugs, protecting Americans from disease and researching new treatments and cures.

Layoff notices began arriving at 5 a.m., workers said, affecting offices responsible for everything from global health to food safety. Senior officials based in the Washington area and Atlanta were reassigned to the Indian Health Service and asked to choose among locations including Alaska, Oklahoma and New Mexico — a tactic to force people out, employees said.

The layoffs and reassignments touch every aspect of the federal Department of Health and Human Services, and are part of what the administration has said is a vast restructuring of the agency. Entire units focused on reproductive health and preventing gun injuries were wiped out. So was a vaccine research program aimed at preventing the next pandemic.

On Tuesday afternoon, Senator Bill Cassidy, Republican of Louisiana and chairman of the Senate health committee, summoned Health Secretary Robert F. Kennedy Jr. to testify about the agency reorganization at a hearing on April 10.

Outside experts and former officials said the loss of expertise was immeasurable. Many described it as a “bloodletting.” Hundreds of people, many carrying handmade signs, gathered in the lobby of a National Cancer Institute building in the Maryland suburbs on Tuesday morning to witness the exodus of fired workers, but were dispersed so they could walk out without fanfare. Some employees, both current and former, were in tears.

But as staff members reeled and comforted one another, Mr. Kennedy posted a video on social media that showed him swearing in the new heads of the Food and Drug Administration, Dr. Martin A. Makary, and the National Institutes of Health, Dr. Jay Bhattacharya.