Christmas Day security was tight at the nation's airports as officials canceled six Air France (search) flights, saying there was evidence terrorists planned a jetliner attack on a U.S. target.

Los Angeles International Airport (search), which handles 150,000 passengers daily during the holidays, remained under its strictest guard since two months after the Sept. 11, 2001 terrorist attacks. Private cars were barred from picking up or dropping off passengers at the curb.

Three flights between Paris and Los Angeles were scratched Wednesday. Two Thursday flights from Los Angeles and one from Paris also were canceled.

American authorities notified France that "two or three" suspicious people, possibly Tunisian nationals, were planning to board the flights, said a spokesman for French Prime Minister Jean-Pierre Raffarin (search).

Many at the Los Angeles airport remained focused on their travels.

"All I care about is getting home," Goran Sobaca said Wednesday at the airport. The 20-year-old student was heading to Belgrade, Serbia-Montenegro to celebrate Christmas with his family but his Air France flight to Paris was canceled. He arranged to reach Paris via Atlanta.

Jaime Anselen, a 79-year-old neuropsychologist from Malibu, said he had to cancel his vacation to Madrid because the alternate flight Air France offered him wouldn't have allowed much time to change planes in Paris.

"They ruined my vacation," Anselen said as he waited for a shuttle back to his car. "I have to cancel because I'm too old to be running through an airport. What kind of vacation is that?"

Still, Anselen said he understood the security concerns.

"I don't blame them," he said.

The airport, one of the busiest in the world, has twice been targeted for attacks in recent years — a foiled bomb plot planned for around New Year's Day 2000, and a shooting at a check-in counter that left three people dead on the Fourth of July last year.

Security around the nation has tightened since President Bush on Sunday raised the national terror alert to orange, its second-highest level.

On Wednesday evening, the Delta Air Lines terminal at New York's La Guardia Airport (search) was temporarily evacuated after a female passenger set off a metal detector and went into the concourse before she could be rescreened, Transportation Safety Administration spokeswoman Amy von Walter said.

Kendall Haynesworth, 29, flying out of Boston's Logan Airport, said the tighter security made her feel safer.

"I feel the U.S. is paying attention. I also think it makes the terrorists think twice," she said.

Elsewhere, travelers were nonchalant as they sipped drinks at La Guardia Airport in New York or browsed for books at Chicago's O'Hare. Many travelers moved quickly through airports, despite heightened security and wet weather.

Officials worried that terrorists might try to use biological, chemical or radiological weapons installed more sensors around urban areas in California and elsewhere to detect dangerous microbes in the air.

The U.S. Coast Guard has upped its surveillance to 24 hours a day at ports such as San Francisco, where foreign merchant ships dock daily.

California Highway Patrol spokesman Tom Marshall said 28 helicopters and planes were flying patrols over electrical grids, aqueducts, major bridges, power plants and state buildings.

Gov. Arnold Schwarzenegger ordered the patrols because of the national orange terror alert, Marshall said.