WASHINGTON – Nearly a week before the air-travel crush for the Thanksgiving holiday, federal security officials struggled Monday to reassure rising numbers of fliers and airline workers who are outraged by new anti-terrorism screening procedures they consider invasive and harmful.

Across the country, passengers simmered over being forced to choose scans by full-body image detectors or probing pat-downs. Top federal security officials said the procedures were safe and necessary sacrifices to ward off terror attacks.

“It’s all about security,” said Homeland Security Secretary Janet Napolitano. “It’s all about everybody recognizing their role.”

Despite officials’ insistence that they had taken care to prepare the American flying public, the flurry of criticism from private citizens to airline pilot groups suggested that Napolitano and other federal officials had been caught off guard.

“Almost to a person, travel managers are concerned that TSA is going too far and without proper procedures and sufficient oversight,” said Kevin Mitchell, chairman of the Business Travel Coalition, an advocacy group representing corporate travel departments. “Travel managers are hearing from their travelers about this virtually on a daily basis.”

Jeffrey Price, an aviation professor at Metropolitan State College of Denver, said two trends are converging: the usual holiday security increases, and the addition of body scanners and new heightened measures stemming from the recent attempted cargo bombings. Also, several airports are short-staffed, which will add to delays, Price said.

Homeland Security and the TSA have moved forcefully to shift airport screening from familiar scanners to full-body detection machines. The new machines show the body’s contours on a computer stationed in a private room removed from the security checkpoints. A person’s face is never shown, and the person’s identity is supposedly not known to the screener reviewing the computer images.

Concerns about privacy and low-level radiation emitted by the machines have led some passengers to refuse screening. Under TSA rules, those who decline must submit to rigorous pat-down inspections that include checks of the inside of travelers’ thighs and buttocks. The American Civil Liberties Union has denounced the machines as a “virtual strip search.”

Douglas R. Laird, a former security director for Northwest Airlines, said it’s the resistance to these measures that will cause the most delays. The new enhanced pat-downs, an alternative to body scanners, take more time — about 2 minutes compared with a 30-second scan. Delays could multiply if many travelers opt for a pat-down or contest the new procedures.

Beyond the scanning process, passengers will also be subject to greater scrutiny of their luggage and personal identification, and stricter enforcement of long-standing rules like the ban on carry-on liquids over 3 ounces.