As the war against Iraq began to unfold Thursday, many cancellations and closures took place across the United States.

Authorities fanned out to power plants, bridges, state capitols and other facilities to shield them against possible retaliatory strikes, all part of Operation Liberty Shield, the plan to fortify the nation against terror.

Though most protections have been in place since the terror alert status was raised to orange Monday night, some states further tightened security Thursday as the war began.

"The first 96 hours of the war is very important, and it deserves special consideration," said Missouri's homeland security adviser, Tim Daniel.

The U.S. Park Police are determined to keep most major monuments and parks open to the public. Sgt. Scott Fear said officers are working extended shifts in the Washington area, San Francisco and New York.

But the White House, perhaps the most recognizable American symbol, was closed to tourists Thursday. And the Washington, D.C., police used a network of closed circuit cameras to monitor activity at landmarks, including the Washington Monument, the Capitol and Union Station.

Sunday's D.C. Marathon was canceled because of security concerns. Race director John Stanley made the decision Wednesday night after his office received more than 1,200 calls and e-mails from worried runners.

More than 6,800 runners from 50 states and 14 countries had registered for the race. A spokesman for the mayor's office said Mayor Anthony Williams is "enormously disappointed" and wasn't consulted before the decision was made.

Despite some changes, many aspects of American life continued in a modified form to reflect the nation at war.

Plans for a subdued Academy Awards ceremony may be the most visible symbol of how conflict with Iraq transformed events at home.

Organizers canceled the splashy red carpet arrivals for celebrities. Barbara Walters' annual pre-Oscar interview was postponed. And the Oscar ceremony — from host Steve Martin's monologue to the celebrity presentations and film-clip montages — will be changed to reflect the nation's mood.

"To do something that will be self-serving or frivolous on a night when our troops are in bloody combat would be absolutely inappropriate," Academy of Motion Picture Arts & Sciences President Frank Pierson said.

The first round of the NCAA men's basketball tournament, scheduled to begin Thursday, was also slightly altered. CBS moved the start of tournament coverage from the network station to cable channel ESPN because of news coverage of the war.

Major League Baseball had already decided earlier in the week that opening the season in Japan wasn't worth the risk and canceled next week's series between Seattle and Oakland.

In Los Angeles, Celine Dion was supposed to get a star on the Hollywood Walk of Fame Friday, but asked to have the ceremony postponed because of the war.

"I do not feel it is appropriate at this time to celebrate this milestone in my career when we are on the brink of war," the Canadian singer said in a statement Wednesday.

Throughout California, a slew of gatherings rearranged their schedules due to war.

The Reno-Tahoe Winter Games Coalition canceled events at Squaw Valley USA ski resort to outline its bid to bring the 2014 Olympics to Lake Tahoe.

The University of California Board of Regents canceled a meeting in San Francisco on Wednesday, and several schools canceled spring break travel plans. In San Diego and Ventura, some schools called off trips to Europe.

In San Francisco, runners in Sunday's "Across the Bay" 12-kilometer road race will be re-routed to avoid the usual scenic route across the Golden Gate Bridge.

In Denver, Colo., the emergency command center, already dealing with a paralyzing blizzard, kept in close contact with the State Patrol, the National Guard and local law enforcement agencies to coordinate a response in case of a retaliatory terrorist attack.

War concerns prompted the Las Vegas Convention and Visitors Authority to suspend promotional television advertisements for at least one week.

Las Vegas-based R&R Partners, which launched a $17.2 million, 20-month ad campaign for the convention authority in January, said Wednesday it will not purchase any television ad slots on behalf of Las Vegas until at least next week.

"Television is going to be overrun with war coverage and that's not necessarily the best venue for us to be getting our message out," said authority spokeswoman Erika Brandvik.

The war against Iraq also prompted organizers of a border terrorism conference to postpone their session. The two-day conference in San Antonio, Texas, had been organized to coordinate law enforcement efforts among agencies in the United States, Texas and Mexico.

Near Houston's Bayport Industrial District, Taylor Lake Village Mayor Natalie O'Neill said she is concerned about a possible retaliatory or terrorist attack because of the city's chemical and petroleum plants nearby and because former President Bush maintains a home in the area.

"We need to be prepared and keep eyes wide open," said O'Neill. "It used to be if we would get involved in a war across (the) ocean, we thought we were safe, but that's not necessarily true. I don't think anyone realized that until 9/11."

The Associated Press contributed to this report.