The White House, meanwhile, tried to reassure Americans that, because of those precautions, they can continue to travel safely.
Lines at security checkpoints moved relatively smoothly on the second day of the new protocols, even though already heavy security restrictions got worse.
Thursday night, British Airways banned carry-on bags from all flights between the United States and Britain, and on Friday, passengers in the U.S. will be subjected to a second security check at their boarding gates. Officials want to prevent anyone from getting onto a plane with liquids that could be used as explosives.
Rather than filling trash cans at security checkpoints with now-banned bottles of makeup, perfume and suntan lotion, travelers were packing those items in their checked luggage.
Thursday saw long lines and confusion as security officials enforced tighter luggage restrictions and a ban on liquids of all types.
Security check lines for domestic flights shrunk to lengths closer to normal Friday morning at Miami International Airport, spokesman Greg Chin said.
Operations at Chicago's O'Hare International Airport were also back to normal, with waits between 30 and 60 minutes, said Transportation Security Administration spokeswoman Lara Uselding.
"Things are running very well for a Friday" at O'Hare, said United Airlines spokeswoman Robin Urbanski.
The new rules were hastily added early Thursday after the alleged terror plot was disrupted in London. It wasn't clear how long they would remain in effect, though Rep. Pete Hoekstra, R-Mich., said the situation "eliminates the days of carry-on baggage."
Frances Fragos Townsend, President Bush's homeland security adviser, said it's safe to fly even as she acknowledged that investigators continue to search for some of the terrorists accused in the plot.
"We are looking for connections between anyone in the United States and the plotters in the U.K., but we don't have any evidence that there is an active threat or cell here," she said on CBS's The Early Show.
Though British officials have arrested two dozen alleged plotters, several suspects remain at large, including the suspected ring leaders of the London plot.
Still, Townsend said people should continue to fly.
"People ought to feel safe about traveling because of all the precautions we're taking," she said.
National Guard troops arrived to back up airport police on both coasts, but no show of force can defeat the type of threat at the center of the London plot. The federal government has worried about so-called liquid bombs since the mid-90s. Various security contractors have been working on detectors that can identify liquids and identify the hazardous ones, reports CBS News correspondent Bob Orr.
But millions of tax dollars, intended for explosive detection, have been diverted to pay the salaries of security screeners. And for now, airports are not equipped to detect liquid bombs or their component parts, Orr reports.
Security was tightest on flights to and from the United Kingdom, with more items banned and additional luggage checks for passengers. Some long lines formed ahead of Miami's early international departures Friday, but they quickly cleared out, Chin said.
Kingsley Veal, 35, a geologist from England, said his Continental flight from London's Heathrow airport to San Francisco was "long and boring" because, under British flight restrictions, he couldn't bring any books or music on board. But he thought the no-carry-on policy should always be in effect.
"If no one's allowed anything, then you'd know, right?" Veal said.
But she said people also didn't appear to be ditching their upcoming travel plans. "We are not seeing any change in bookings ... nothing out of ordinary," she said.
At Seattle-Tacoma International Airport, about 3,700 of an estimated 51,000 travelers missed their flights because of lines and delays on Thursday, when 164 flights were delayed, said airport spokesman Bob Parker.
Traffic was moving more smoothly at New York's major airports on Friday, where the morning flight delays were generally no more than 15 minutes, said Tiffany Townsend, a spokeswoman for the Port Authority of New York and New Jersey.
From London to Los Angeles, travelers on Thursday had found themselves unpacking carry-on bags on the floor in the terminals. Some tried to squeeze makeup, sunscreen and other toiletries into their checked baggage, where liquids were permissible. Others filled up the bins at security checkpoints, abandoning everything from nail polish to a bottle of tequila.
"I literally lost about $50 or $60 worth of things we were told to throw out," said Terry Asbury, who flew into Cincinnati from Albuquerque, N.M., and found herself dumping all her cosmetics.
At Minneapolis-St. Paul International Airport, Kristin Reinke, of Champlin, Minn., complained to her husband, Mike Reinke, "I just threw out $34 worth of hand lotion."
He was sympathetic, but accepted the Transportation Security Administration's reasons for the ban.
"What are you going to do?" he said. "I guess you have to be safe."
Amanda Volz, a TSA screener in Minneapolis, said she hoped more travelers would take that attitude Friday.
"There's some moaning and groaning, and a few people who get angry, but once you explain it to them, they are more lenient about giving it up," Volz said. "You just try to make them understand that it's for their safety."
The ban on liquids and gels covered such things as shampoo, toothpaste, contact lens solution, perfume and water bottles. The only exceptions were for baby formula and medications, which had to be presented for inspection at security checkpoints. Liquids are allowed in checked bags because those suitcases are screened for explosives and are stowed in the cargo hold beyond passengers' reach.