The Secret Service barred pedestrians from Pennsylvania Avenue in front of the White House, District health officials stepped up the pace of smallpox vaccinations and authorities imposed new restrictions on private planes flying over the capital as the Washington area braced for the start of war against Iraq.

Security also was tightened across the country, with more inspections at bridges and airports, increased patrols at ports and nuclear power plants and new limitations on flights over sports stadiums and the Walt Disney World and Disneyland theme parks.

In the capital region, Maryland, Virginia and District police increased surveillance at tunnels, bridges and other sensitive locations in reaction to the federal government's decision Monday to raise its terrorist threat level to orange. Authorities also reinstated occasional searches of cars entering the region's three airports.

D.C. Mayor Anthony A. Williams (D) urged residents to be on the alert for suspicious activity and announced that visitors to all District government buildings will need to show identification and should expect delays. He said that at this point he did not expect closures of critical commuter routes near landmarks such as the U.S. Capitol and the White House.

"We cannot panic. We must be vigilant," Williams said at a late-afternoon news conference. "We've got to be an open city. We've got to be a safe city. We can manage it."

But the mayor also said he was concerned that the heightened security might discourage people from visiting Washington just as it is getting ready to host the annual National Cherry Blossom Festival, and there were already signs yesterday of an adverse impact on the city's tourism industry.

Fairfax County's school system, the largest in the region, canceled all student field trips to the District and New York City until further notice. And the Secret Service's new restrictions along Pennsylvania Avenue NW, which took effect yesterday, will eliminate a tradition cherished by many tourists: the taking of their photo on the sidewalk in front of the White House's wrought-iron fence and columned front portico.

"Our nation -- behind bars," said Dan Fetzer, a tourist from Oak Ridge, Tenn., who had come to the White House with his wife and 8-year-old son yesterday for the classic photo and had to settle for a snapshot from across the street in Lafayette Square. "It's kind of sad for our country to be responding like this."

Pedestrians are now banned on Pennsylvania Avenue NW between 15th and 17th streets NW; E Street NW between 15th and 17th streets; and the sidewalks on 15th Street and 17th Street adjacent to the U.S. Treasury building and Eisenhower Executive Office Building, respectively. The Secret Service issued a brief statement saying it had ordered the expanded security perimeter "in cooperation with our law enforcement counterparts" without further explanation.

Victoria Isley, a spokeswoman for the Washington DC Convention and Tourism Corp., encouraged concerned travelers to visit its Web site, www.washington.org, for the latest travel updates. "Washington, D.C., is open for business, and we will continue to welcome visitors here," she said.

In other measures linked to the heightened terrorist alert, the D.C. Department of Health speeded up its pilot program to inoculate a team of 30 health workers against smallpox, planning to give them the shots this week. The vaccinations will be given to 300 to 400 hospital workers citywide beginning in April, department spokesman Briant K. Coleman said.

The Federal Aviation Administration announced that all private pilots flying within a 35-mile radius of Reagan National, Dulles International and Baltimore Washington International airports will be required to file a flight plan before taking off.

Returning aircraft based at three Prince George's County airports -- College Park, Potomac and Washington Executive/Hyde -- must first land at Tipton Field at Fort Meade and be screened by Transportation Security Administration officials before going to their final destination, FAA spokesman William Shumann said.

Maryland and Virginia officials said they were taking the same precautions as they did during last month's Orange Alert, including increasing patrols at sensitive locations and requiring police officers to have ready access to their protective equipment.

Maryland Gov. Robert L. Ehrlich Jr. (R) urged residents to inform homeland security officials of any suspicious activity by calling 800-492-TIPS. And Virginia Gov. Mark R. Warner (D) said, "We're prepared for any eventuality." He spoke to reporters outside his office in the state Capitol, which was closed to tourists, except previously scheduled charter tours, because of the Orange Alert. Both governors and the D.C. mayor said there was no specific intelligence singling out their jurisdictions as possible terrorist targets.

Fairfax schools, in addition to the ban on field trips to Washington and New York, barred sports teams from traveling to those cities because of concerns over terrorism. School officials said questions about refunds should be directed to trip organizers or companies sponsoring trips.

Dozens of political science students at Herndon High School felt the impact immediately, as they were unable to report to internships on Capitol Hill.

"Nobody went because everybody was really afraid," said Houda Kamoun, 18, a senior and intern for Rep. Gregory Meeks (D-N.Y.). "We have to take it on a day-by-day basis."

Students in her political science class, she said, generally carpool every other day to the internships. Yesterday, her teacher told the students he is trying to figure out what they can do instead.

Staff writers Sylvia Moreno, Patricia Davis, Craig Timberg, R.H. Melton, Lori Montgomery, Petula Dvorak, S. Mitra Kalita and Josh White contributed to this report.

A heavily armed security officer patrols the north lawn of the White House while pedestrians observe from the sidewalk and Lafayette Square.Enrique Herazo, 21, works the line on a Coast Guard boat after cruising the Potomac River for a daily security check.