PHILADELPHIA - Crowded airports and long lines are to be expected when traveling on Thanksgiving. But air travel this holiday could be more unpredictable than ever.

A mini-revolt is erupting across the country, among groups ranging from pilot unions to civil libertarians to ordinary fliers, over 385 new advanced-imaging scanners now part of Homeland Security screening in 68 airports, including one scanner at Philadelphia International Airport, in Terminal F.

A grass-roots movement has sprung up on the Internet that calls for a nationwide "opt-out" protest on Wednesday, one of the busiest travel days of the year. It urges air travelers to refuse to be scanned. Those who decline will be subjected to a full-body pat-down - which critics liken to a virtual strip search. (Some who have experienced the pat-down, however, say it's no big deal.)

A 7 p.m. opt-out rally is planned at Philadelphia airport, organized by James Babb, an Eagleville, Montgomery County, businessman.

The idea is to cause havoc and delay, since the pat-downs take much longer than body scans.

Will the hoopla result in missed flights and foregone turkey dinners? It's doubtful. Most people just want to get where they are going.

With the holiday season here, and more Americans expected to fly - 24 million, up 3.5 percent, over the Thanksgiving travel period - the sprint to the plane could be fraught with gridlock.

Philadelphia airport officials and the Transportation Security Administration say they will be ready, with adequate staffing to manage increased passenger volumes and any renegade demonstrators.

"We always take an all-hands-on-deck approach to the holidays," said TSA spokeswoman Ann Davis. "All the checkpoints will be fully staffed. We can't really speculate on how the opt-out-day movement will impact wait times, but we are aware of it and we are prepared."

Add to the new security measures jam-packed planes, and passengers cramming more into their carry-on bags to avoid checked-luggage fees - and not all bags fitting into overhead bins - airports and airlines have a word of caution: Give yourself plenty of time to go through security.

Philadelphia airport expects 700,000 travelers to pass through, beginning Sunday through Monday, Nov. 29.

The part of screening that takes the longest is not walking through a metal detector or standing in the imaging machine a few seconds, but emptying pockets of change, removing shoes, and screening carry-on bags.

Holiday travel attracts the infrequent, once-a-year passenger who may be unfamiliar with security protocol and guidelines for carry-on items.

"They are coming to the airport, packing as much into their carry-on bags as possible to avoid the checked-baggage fee," said Kevin Mitchell, chairman of the Business Travel Coalition. "That causes the TSA security personnel to do more hand searches. Consequently, it makes the throughput at the checkpoint slower, and it causes problems at the gate."

Tensions at airports could be higher this Thanksgiving if the opt-out boycotts materialize to any extent. "The question then becomes: What small percentage of renegades does it take to choke the system?" Mitchell said.

Babb, spearheading the rally here, said: "We will be educating passengers about what they may be in store for with the new scanners.

"It's a grass-roots response to the invasive procedures that the TSA is now imposing, including radiation strip searches and the sexual gropings."

Asked about rumors that some protesters may strip off their clothes to express their opposition to full-body scans, Babb laughed - and winked.

"No comment. I'll say for the record that I'm not," Babb said. "I have heard certain things that people are talking about, different ideas. This movement is full of creativity, outside-the-box thinking."

Among those planning to be part of the Philadelphia opt-out are members of Drexel University's Student Liberty Front.

"Mostly, we're tired of the government trying to say it can infringe upon our rights of privacy," said student Deanna Quinones, 22, of Hamilton, N.J., outside the airport's Terminal F last week. "With the body scanners, they can completely see us. If we opt out, we get our body felt instead.

"Both ways, it seems like sexual assault to me. If it was anyone else doing it, it would be considered pornography or harassment."

Drexel student Pericles Miarchos, 24, of Long Island, N.Y., said: "Since 9/11, it's been an increase of abuses, from the Patriot Act to the Foreign Intelligence Surveillance Act. Now we've got TSA agents taking nude photographs of us, patting us down. My chance of dying in a terrorist attack on a plane are probably one in five million. I'm more in danger from asteroids, or stray baseballs."

A CBS News poll released last week showed 81 percent of Americans believe airports should use full-body scanners to screen passengers.

Passengers standing in line to be screened in Terminal F last week expressed various views. None said they would refuse the scan - and none said they would not fly.

"It doesn't bother me because I just need to fly," said Tom Bingle, a tour operator from Breckenridge, Colo. "The question is: Do you feel safer flying now? Psychologically, I feel safer. I have had a pat-down, but maybe not a more thorough one. It doesn't matter. I don't care."

Barbara Minor, an engineer from Elkton, Md., on her way to Toronto, said the new pat-downs "sound excessive" but will not stop her from flying. "I have to fly for work. I would do the scan, just because it's quicker. I am not worried about radiation, no."

Frequent flier Steve Lapin of Elkins Park wanted to know what the fuss was about, and, returning from a business trip in Pittsburgh last week, opted out of the scan in order to get a pat-down.

"I have no problem with the technology, but I wanted to see what the pat-down was like," he said. A TSA agent took him aside, and instructed him to put out his arms.

"He told me what he was going to do: check the collar of my shirt, my sides, the waistband of my pants, my legs, halfway up my thighs, front and back. And that's what he did. He had gloves on," Lapin recalled.

"He went to the back of my collar, down the front of my shirt, rubbed his hand on the inside of my waistband, where the belt loops are, down the front of my leg. Then the same thing on the back - down to the ankles and up the ankles, to halfway up my thigh. That was it. Nowhere near my private parts - not even close.

"My reaction was: 'Is that it?' The officer said, 'That's it. Go get your stuff.' "

How to Prepare

These steps can help during the holiday rush:

Check in on the airline's website up to 24 hours before your flight, and print your boarding pass at home.

If you plan to check a bag, prepay online during flight check-in to save time at the ticket counter.

Pack light. Make sure your carry-on bag fits easily under the seat or in an overhead bin.

Allow plenty of time. Philadelphia International Airport recommends arriving two hours before a domestic flight and three hours before an international trip.

Have your boarding pass and ID available, in your hand or top pocket.

Put liquids in a clear plastic zip-top bag, on top of your carry-on. Adhere to the 3-1-1-rule: 3-ounce or smaller containers of liquids or gels in a 1-quart clear plastic zip-top bag, 1 per passenger.

Send jackets through the X-ray machine. Security agents routinely pat down passengers in bulky clothing, including fleeces and hooded sweatshirts.

Remove keys, loose change, belts, and jewelry. If the metal detector goes off, a pat-down will follow.

Take laptops, video games with consoles, and cameras with cassettes from cases and put in a separate bin for screening.

Do not wrap gifts. Security may need to inspect the package.