Fears of retaliation against Americans since the U.S. bombing of Libya already have had a staggering impact on travel plans from children's school excursions to business and vacation trips.

American travel abroad will fall by as much as 80 percent this summer, according to tourism experts. Some U.S. airlines have reacted by increasing security, and travel agents are pushing destinations such as Moscow, China, Ireland and Canada instead of Mediterranean cities.

"Things are bad. Let me tell you, things couldn't get much worse," said Dan Bohan, vice president of Omega World Travel. "Everybody's canceling all their European trips."

Bohan said Omega, with 32 offices and about $10 million in revenue from international travel a year, could sustain a loss of as much as 50 percent of its foreign travel business.

Trips closer to home also are affected. At Olde Creek Elementary School in Fairfax County yesterday, 50 second graders who had been scheduled to take a field trip to the Corcoran Gallery were disappointed.

"In view of the international situation, field trips to the District of Columbia are being discouraged," said a letter from the school to parents.

"Since the Libya situation, it appears that Europe and Mediterranean travel is down around 75 to 80 percent," said Willard R. Brown, head of travel agency services for the American Automobile Association.

Travel agents and industry experts said yesterday that U.S. airlines with heavy concentrations in some European routes are suffering because of the cancellations.

"Certainly the two major U.S. carriers . . . , Pan Am and Trans World Airlines, will be hurt very badly," said George James, president of Airline Economics Inc. and an analyst of the industry."It could mean increased traffic for their foreign competitors."

Spokesmen for those two airlines would not comment on how much traffic to the Mediterranean is down, although both said that it was reasonable to assume it is down. "Obviously, we're seeing some of that reaction," said a Pan Am spokesman.

"We're not seeing a massive rash of cancellations," he said. "What we're seeing is a shift from southern Europe to northern and eastern Europe. In particular, Moscow is selling like crazy. Advance bookings for Ireland are incredibly strong."

Pan Am officials said this week that they would ask the Department of Transportation to approve a $5 surcharge on every ticket sold for trans-Atlantic flights to help defray the costs of increased security.

Security costs in Rome alone will be about $1 million this year, up from about $650,000 last year, a spokesman said. "That surcharge won't come close to paying security costs," he said. Pan Am flies to 32 European destinations.

TWA officials said they are considering a similar surcharge.

Earlier this week, the Federal Aviation Administration introduced new restrictions on air carriers traveling to Europe. The FAA has declined specific comment on the restrictions but, according to one Transportation Department official, they include increased inspections of baggage and more systematic surveillance of airline employes.

Eastern Airlines officials said yesterday they had initiated additional security measures, including possible physical searches of passengers, searches of carry-on and checked luggage, and the hiring of additional security personnel.

Officials declined to give the locations for increased security, saying that might undermine the company's safety plans.

At American airports, the FAA is responsible for establishing and monitoring security, which is carried out and paid for by airport operators and the airlines.

Rodney Wallis, director of security for the International Air Travel Association, called terrorist activities on airplanes "a new and horrifying problem. Its implications are profound for the traveling public."

One airline industry official said that during this high-risk period, one possible approach would be for the U.S. government to provide American air carriers with additional surveillance in airports in Europe. He noted that the Israeli government provides specially trained agents to bolster security for that country's El Al airline at airports around the world.

However, other officials in the airline industry have questioned the feasibility of implementing such methods.

"I'm tired of hearing 'Why don't we use the El Al methods?' " said James Murphy of the Air Transport Association, which represents U.S. airlines. "If you're in a state of war and you only have 20 airplanes and the state owns the airport, you could do that. We have 15,000 flights a day, 3,000 planes carrying 1 million passengers a day -- a far cry from El Al.

"It sickens me to hear people say, ' Fly Swiss Air' . . . . U.S. security is excellent."

However, airline reservations clerks said yesterday that the public is reacting nervously, switching to foreign carriers and going out of its way to avoid certain areas.

"Some people are flying into Switzerland or Germany, and taking the train to Rome," said Bohan.

Studies show that even before the Libya bombing, Americans had been discouraged from traveling abroad by incidents such as the attacks on airports in Rome and Vienna.

About 1.8 million Americans -- or 35 percent of U.S. travelers with foreign reservations -- changed their plans in February, the latest month for which figures are available, according to the U.S. Travel Data Center. Of that number, 1.4 million decided against traveling abroad at all, opting to switch to a domestic destination or cancel altogether, according to the agency.

"It's an absolute shame, a tragedy," said Donald McSullivan, of the European Travel Commission. "This has all been caused by Qaddafi, and it's a shame that one man has been able to stop Americans from traveling abroad."

McSullivan, who represents a commission of 23 tourist boards that exists to promote European travel, said he expects to see tourism drop by as much as 30 percent in some countries.

Other travel officials said that as early as last fall, the industry expected trips to Europe to be down this year. A weaker U.S. dollar and increased travel in the United States and Canada were cited as reasons.

The travel industry is not the only victim of the widespread fear of terrorism. D.C police said there had been seven bomb scares in the city by late afternoon, down from Thursday's 11 reports.

"I think it's scary, and I think Washington takes this a little lightly," said Jody Fine, an employe of the American Federation of State, County and Municipal Employes, as she waited to return to her office building after a bomb threat.

Local schools and colleges reported yesterday that summer trips to Europe and the Middle East were under-enrolled or canceled.