Travellers vowing to boycott controversial see-through airport scanners in the US threatened chaos on the busiest US air travel day.
Americans have voiced outrage over the new, personally invasive security searches and threatened airport protests that could snarl up the system tomorrow - the peak of Thanksgiving holiday travel.
Body scans take as little as 10 seconds, but people who refuse to undergo the scan must submit to a full pat-down, which takes much longer. That could cause a cascade of delays at dozens of major airports, including those in New York, Los Angeles, Chicago and Atlanta.
The Obama administration and security chiefs acknowledged public anger but emphasised the need to keep travellers safe from potential terror attacks in the sky.
The uproar is over new procedures implemented by the Transportation Security Administration, which was created after the September 11 2001 terror attacks and has toughened airline security significantly since.
The new checks include body-scanning devices at about 70 US airports that produce virtually nude, although unrecognisable, images of travellers.
Those who refuse the scan are allowed to undergo pat-down body searches, but those can include the touching of genitals through clothing.
Cable television outlets, internet sites and blogs have been dominated by stories about the procedures, some highlighting the indignities they have caused for travellers.
A bladder cancer patient from Michigan who wears a catheter bag said a security agent at a Detroit airport patted him down so roughly, the bag spilled urine on his clothing.
Tom Sawyer, a 61-year-old retired special education teacher, said the experience left him in tears before he caught a flight to Orlando, Florida.
"I was absolutely humiliated. I couldn't even speak," he said.
Transportation Security Administration chief John Pistole said he was concerned about people such as Mr Sawyer who have had uncomfortable experiences with agents performing the body searches.
Mr Pistole also said he understood public anger about privacy but stressed that a relatively small proportion of the 34 million people who had flown since the new procedures came in had body pat-downs.
White House spokesman Robert Gibbs said yesterday that the government was "desperately" trying to balance procedures that maximised security and minimised invasiveness.
He said President Barack Obama's highest priority during the holiday season "is to ensure that when you or I or others get on to an airplane, that we can feel reasonably sure that we can travel safely".
That however, may not dissipate a continuing internet campaign to boycott body scans.
A National Opt-Out Day is planned for tomorrow to coincide with the busiest travel day of the year, when Americans leave home in huge numbers for the Thanksgiving holiday.
"Just one or two recalcitrant passengers at an airport is all it takes to cause huge delays," said Paul Ruden, a spokesman for the American Society of Travel Agents.
Reminding Americans of the alleged Christmas Day bombing attempt by a Nigerian with explosives in his underwear to bring down an Amsterdam-to-Detroit flight, Mr Pistole said: "We all wish we lived in a world where security procedures at airports weren't necessary, but that just isn't the case."
US secretary of state Hillary Clinton said she thought "everyone, including our security experts, are looking for ways to diminish the impact on the travelling public" and that "striking the right balance is what this is about".
She said she would not like to submit to a security pat-down.
"Not if I could avoid it. No. I mean, who would?" Mrs Clinton said.