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. Some senior officials based in the Washington, D.C., area were reassigned to the Indian Health Service and asked to choose between 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 offices, including a vaccine research program aimed at preventing the next pandemic, were wiped out.

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, to witness the exodus of fired employees. Some of the observers and the former employees cried.

But as staff members reeled and comforted one another, Health Secretary Robert F. Kennedy Jr. 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.

“Welcome aboard,” Mr. Kennedy said. “The revolution begins today.”

The cuts were intended to fulfill Mr. Kennedy’s plan, announced last week, to shrink his department from 82,000 to 62,000 employees. Tuesday’s layoffs affected 10,000 employees, on top of 10,000 who had already been fired or left voluntarily. The department did not respond to a request for comment on the record.

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