The nation's capital became a city at war yesterday, as police guarded every major bridge and roadway, war protesters worked hard to disrupt traffic, and residents gathered to watch television reports about the conflict with Iraq -- and to remember the military personnel whose lives are on the line.

All 3,600 D.C. police officers began working 12-hour shifts, tightening security across the city. Protesters created one early morning traffic jam, at the Key Bridge, and then held scattered demonstrations and marches throughout the day. Some area high school students walked out in protest of the war, while others drilled their teachers about the meaning of the bombings in a faraway country.

At area airports and in office buildings, houses of worship and private homes, there was no escaping the realization that the nation has embarked on a dangerous, unpredictable mission -- and that Washington, as the seat of power, has seldom been more vulnerable.

It made for a study in contrasts: Passengers on a Yellow Line Metro train were so jumpy yesterday that they momentarily panicked when a minor malfunction caused a popping sound that some feared was a bomb, while shoppers at the mall serenely went about their business. Hours after D.C. Mayor Anthony A. Williams (D) reassured everyone that the National Cherry Blossom Festival will unfold on schedule beginning tomorrow, officials announced that public tours of the U.S. Capitol will be suspended indefinitely, beginning today, for security reasons.

Despite the mixed messages, area officials emphasized that life would continue as normally as possible under the wartime conditions, even inviting tourists to take advantage of the attractions while there are relatively few crowds.

"Washington, D.C., is very much open for business," said Peter G. LaPorte, director of the D.C. Emergency Management Agency, "and folks can come and enjoy the fact that there aren't so many lines at the Smithsonian. There are plenty of places to go -- it's a great opportunity to come to D.C."

Antiwar demonstrators apparently had already gotten the word, taking to the streets of Washington to voice their opposition to the U.S. military attacks on Iraq. For weeks, they had been preparing for what they called "Day X" -- the day after an attack -- but leaders finalized plans only Wednesday night.

Unlike previous Washington demonstrations, yesterday's actions were scattered throughout the city and drew numbers in the hundreds instead of the thousands. But the tactics emphasized what organizers call "direct action," as activists briefly blocked morning commuter traffic on the Key Bridge with their bodies and an abandoned minivan.

"I want to make a statement," said Gail Taylor, 24, of the District, shortly after she linked arms with about 30 other activists stretching the length of the bridge. "I think the war is unjust. It's illegal."

That was the refrain heard from Dupont Circle to the White House, from high school student to labor organizer. Those who oppose the conflict seized an opportunity to speak out, to display their dissent to the war -- and to challenge authority.

In a soaking rain, an estimated 150 students, some carrying placards and giving the peace sign, walked out of Alexandria's T.C. Williams High School to protest the start of the war. At Montgomery Blair High School in Silver Spring, more than 1,000 students shouted the refrain, "Hey hey, ho ho, we won't kill for Texaco." At the District's Woodrow Wilson Senior High, handfuls of student protesters walked out of school and sat in the hallways during classes, risking punishment.

Wilson Principal Stephen Tarason said he admonished the students to stay in class, adding that he has not decided how to deal with those who disobeyed. First-time offenders, he said, may be subject to detention, and repeat offenders could face suspension. "It's my responsibility as a principal to make sure kids are in school," he said.

At George Mason University, art professor Suzanne Carbonneau and two other faculty members braved the chill rain to distribute antiwar fliers outside GMU's Johnson Center. "Yes, we're officially at war," Carbonneau shouted. "Do something about it."

For some, the answer was prayer. Although people did not flood houses of worship as they did after the terrorist attacks Sept. 11, 2001, congregations across the area held impromptu services or incorporated special prayers into daily Masses or Lenten gatherings.

"Oh God . . . help us pray in the midst of our struggle for justice and truth to confront one another without hatred or bitterness," the Rev. Peter Grandell, liturgist at the Washington National Cathedral, said at a service held in Memorial Chapel, which is dedicated to the memory of U.S. men and women who have served in the military. "Open our hearts, and the hearts of those who think of themselves as our enemies, to your ways of peace, that with your help we may work together with mutual forbearance and respect."

During the noontime Eucharist at historic St. John's Episcopal Church -- described as "the Church of the Presidents" because it has hosted every chief executive during the past 188 years -- the sound of antiwar chants from a group of 100 protesters outside on H Street NW echoed dimly within the chapel.

Some churches announced plans to remain open 24 hours a day until the war ends, so people can step inside and pray.

In many ways, Washington became a 24-hour city with the opening of the war. At the Pentagon, the nerve center of the conflict, the parking lot is full night and day. Police are guarding sensitive federal and diplomatic facilities while maintaining larger-than-usual neighborhood patrols.

A system of video surveillance cameras also has been activated, and stepped-up truck and other inspections are part of a daily fine-tuning of operations. At a news conference, D.C. Police Chief Charles H. Ramsey said "civil disturbance" platoons have been organized and would be waiting in unmarked vehicles -- usually white vans -- to deal with protests.

"It's nothing we haven't seen before," he said. "And it's nothing we haven't handled before."

City officials vented their frustration at organizers of the D.C. Marathon, who canceled the race after months of planning without discussing it with them. The move flew in the face of a concerted campaign by city government and business leaders to keep the war from deflating the local economy. Organizers of the upcoming World Figure Skating Championships and the Cherry Blossom Festival were praised for their plans to continue the events.

At Reagan National Airport, some people were intent on leaving town, but they were calm about it. Indeed, as travelers nosed through bookshops, sat in restaurants or tapped away at laptop computers while waiting for flights, nothing seemed any different, they said. Many were openly defiant at the suggestion of changing their travel plans.

"We never even thought about changing our minds -- and we plan to go on our cruise next week, too," said Harriet Sells, who with her husband and grandson was on her way back to Jackson, Tenn., after a four-day vacation here. "It's not that we're not thinking about war; we just feel as if things here will be okay."

For area military families, the stakes were obviously higher. Heather Wirtanen of Bel Air gathered her two daughters, ages 5 and 7, close and tried not to worry about her husband, Chief Warrant Officer Robert J. Wirtanen of the Marine Corps Reserve, who is on the front lines and engaged in battle.

"The girls don't understand the magnitude, that he's in a life-and-death situation," said Wirtanen, 38. "They just miss their daddy a lot and are concerned for his safety. I keep reassuring them that . . . he's done it before and came home just fine. It's almost a false sense of security because although he's done it before, it's a different campaign now."

From the Jefferson Memorial, a U.S. Park Police officer surveys the city. Law enforcement agencies are guarding sensitive facilities, and D.C. police expanded neighborhood patrols.D.C. Police Officer Dennis Hance, in his boat, examines the underside of the Key Bridge for anything amiss. Activists briefly blocked traffic on the span with their bodies and a minivan.