An hour of terror changed everything.

Far from the World Trade Center or the Pentagon, Florida shut down its state universities yesterday. San Francisco closed its schools, as well as the TransAmerica building and pedestrian access to the Golden Gate Bridge. Major league baseball games were canceled.

The popular, needlelike Stratosphere tower on the north end of the Las Vegas strip was closed; so was the Paris casino's mock Eiffel Tower. University of Virginia psychologist Dewey Cornell canceled his lecture on student threats and violence inside the schools -- so his audience of principals could go back to their schools to deal with the violence outside.

"You just thought America was the safest country," said Jesse Strauss, a 13-year-old eighth-grader at Pelham Middle School, a Manhattan suburb. His mother added, "Our world as we know it isn't going to return to normal for a long time."

Yesterday's terrorism darkened, marked and forever altered the way Americans live their lives.

"We are going to have to learn what a lot of other countries have gone through: to manage fear at a cultural and national level," said Charles Figley, a professor of trauma psychology at Florida State University. "We're getting a lesson in the way fear works."

In a country long proud and even boastful of its openness -- a country where an ordinary citizen can stroll through the U.S. Capitol unescorted -- the terrorist attacks are likely to force Americans to watch their steps and look over their shoulders. We already do a lot of that. Metal detectors now mark the front door of many government buildings, and security guards are a fixture in the lobby of most large office buildings.

But tightening still further carries its own danger of allowing terrorists to change a fundamental of American life. "It's a test of us," said Fred Dutton, a former aide to John and Robert Kennedy who now represents the government of Saudi Arabia in Washington. "Are we going to become insecure, and feel the need to have a less open, government-controlled society?"

"The worst thing we could do is say, `This is the way things are going to be from now on,' " said Robert Butterworth, a Los Angeles psychologist who heads a disaster response network. Avoiding crowds, popular events and high profile venues like Disneyland or Sea World -- which also closed yesterday -- is a logical response, but we also "have to figure out constructive things to do," he insists.

Retaliation is another logical response. Indeed, President Bush promised as much. In an example of the country's mood, a scrawled sign outside a blood bank in New York ordered, "Mr. Bush, Bomb the bastards now."

But retaliation carries the risk of setting off a tightening spiral of violence and counterviolence not unlike the Middle East or Northern Ireland. Unlike countries that have had to learn to live with violence, "We are new at this," said Florida's Dr. Figley, who heads a project that has trained trauma teams in Yugoslavia. "My fear is we will overreach and make things worse rather than better by retribution, revenge, racism and marginalizing ethnic groups."

That fear is especially true for Jews and Arabs. In Brookline, Mass., Congregation Kehillath Israel, like many other Jewish congregations, plans to double the security detail at next week's services for Rosh Hashanah, the Jewish New Year, and the Yom Kippur holy day 10 days later. Police cars will be stationed outside, and uniformed and plainclothes police inside.

"I think I now understand what it is like to live in Jerusalem," said the congregation's rabbi, William Hamilton.

Meanwhile, the city of Dearborn, Mich., moved to ensure there isn't a backlash against the city's large Arab-American population by setting up an emergency operations center and putting 22 extra police officers on patrol.

Fear of terrorism is likely to lead Americans to tolerate more government surveillance -- such as overhead video cameras at sporting events -- than they have to date. "It's very likely in the wake of today's events that we're going to see a greater acceptance on the public's part -- and on the court's part -- to approve certain kinds of police tactics," said William Stuntz, a Harvard Law School professor.

"Today represents a real change in the world," he added. "It's not possible ever to think of these issues the same way."

In Redding, Calif., the chief of police, Robert P. Blankenship, agreed. "We're not going to be as comfortable and as secure as we once were. Looking at the TV, it's obvious now that we're vulnerable," he said.

Stepping up security isn't always possible, though. Fairfax, Va., already posts police officers in its secondary schools; unarmed security officers patrol the district; school doors are locked, teachers and staff wear identity badges. The effectiveness of metal detectors and surveillance cameras isn't proved, and anyway, they "create in kids the sense of a jail," said Daniel Domenech, the superintendent.

Inner-city schools have spent heavily on security technology in the past decade; the Houston school district even has its own SWAT squad. School security has long looked inward for a threat -- to students carrying weapons or picking fights. But rising violence from the outside -- from disgruntled parents or former employees -- is drawing increased attention.

In the wake of the events yesterday, much of the U.S. was closed down -- the federal government, schools, airports, the Hoover Dam near Las Vegas and the 47-story Bank of America building in downtown Miami. Also shuttered were the International Monetary Fund and the World Bank; their fall meetings, scheduled for later this month and a planned target of antiglobalization protests, may be canceled, a bank official said. Other institutions and facilities also will reopen amid greater security, resulting in increased frustration and delays. How to explain the day's inexplicable events to their children will be a huge dilemma for parents. "You're not going to be able to keep this one under wraps," said Dr. Butterworth, the trauma psychologist. But he warned against using the tragedy as a teachable moment -- a common response in the schools to huge national developments -- and overwhelming children.

A further fear is the possibility of copycat incidents that often follow acts of highly publicized violence. Some people "deal with their fears by making other people afraid," said University of Virginia's Dr. Cornell. Indeed, a New York school was evacuated shortly after the planes hit the World Trade Center tower because of a bomb threat. And in Las Vegas, 30,000 people at the International Banking Expo were turned away from the city's convention center after a bomb threat called in from a pay phone on the center's premises.

Maxine Boarts, 71, a real-estate agent from Pittsburgh on a weeklong vacation in Las Vegas wasn't planning to leave until Friday, but is worried about getting a flight home -- "if we're not afraid to" get on a plane then. Watching TV from a bar on Bally's casino floor, she said she and five companions considered renting a car to drive home should they need to, but couldn't find a car to rent. It would be a multiday car trip, "but we'd be alive when we get there."

Ms. Boarts wondered if the events will disrupt her grandson's wedding plans next June, but is more concerned about the effect this will have on the nation's psychology. "We'll look at people so differently now," she said. "We're an open people. We're the kind that would talk to anyone. Now, it'll take a second thought."

A few things didn't change yesterday. Gambling at nearly all Las Vegas casinos continued at near normal volumes, although many gamblers watched CNN as closely as their cards. Merrill Lynch & Co. pressed ahead with a media and entertainment conference for about 500 investors at the Ritz-Carlton Hotel in Pasadena, Calif., after heated argument in the lobby between those Merrill officials who wanted to cancel it and Jessica Reif Cohen, a Merrill first vice president, who didn't.

And Americans, as they have in past moments of shared national tragedy, rolled up their collective sleeves. So many volunteers showed up at a Rockville Centre, N.Y., blood bank that overwhelmed staffers began handing out numbers, then turning away donors with anything but O-negative blood, which is accepted by any recipient. Nonetheless, dozens of would-be donors sat in a line of folding chairs that snaked around the building, waiting their turn.