D.C. police officers found themselves walking surveillance beats around their own police stations yesterday; the Loudoun County School Board voted to cancel all school-sponsored trips abroad this summer, and persons whom police later called "pranksters" phoned bomb threats to private and public buildings as Washington felt its own brand of tremors from bombing raids in Libya.

While security officials increased their vigilance and their manpower, residents and tourists also reacted to the bombings with everything from nervousness to stiff-upper- lipped patriotism, some of it mixed with a bit of humor.

Gary White, an auto worker from Michigan, admitted to some jitters and said he believed Washington must be "at the top of the list" of Libyan leader Muammar Qaddafi's targets.

But Angie Davis, a saleswoman at The Shops at National Place, said Qaddafi may be "crazy, but I think he's really scared to come here. We're too powerful a nation."

At a bus stop in Northwest, as two police cars careened around a corner, their sirens blaring, one young woman remarked, "I hope the Libyans haven't bombed something."

And in Bethesda, when the garbage wasn't picked up on time, one woman told her neighbor, "It looks like our trash trucks have gone to war" -- an allusion to the trucks that have been used in recent years at the White House and the Capitol to block entrances against possible terrorist attacks. It was a false alarm. The trash was collected by afternoon.

In 1983, when an explosive-laden truck killed more than 240 American servicemen in Beirut and a bomb exploded inside the Capitol, Washington began protecting itself behind concrete barriers and metal detectors as never before. Yesterday, official Washington hunkered down once again with a renewed sense of awareness.

On Capitol Hill, where security already is tight, Senate Sergeant-at-Arms Ernest Garcia said he was using "no new machinery" and "no additional troops" but added, "We are on heightened security alert status . . . . We are not assuming something is done. We check and double-check and triple-check."

Maj. Richard Cusick of the U.S. Park Police, which patrols park lands and national monuments, reported increased patrols and surveillance and said that officers may be reassigned to priority areas downtown.

The D.C. police department's elite emergency response team, trained in hostage negotiations and special tactics, was on round-the-clock alert, a source said. Late Monday, all D.C. police facilities were ordered to "secure their stations" and to set up special foot patrols, called "short beats," around their perimeters, sources said.

At least 13 telephoned bomb threats were received yesterday by a variety of local businesses, newspapers and government buildings, according to authorities.

Police said there were no unusual incidents at any of the locations and attributed the calls to pranksters.

While security officials gave typically tight-lipped responses to questions -- "Countermeasures are not countermeasures once the countermeasures are made public," said one spokesman -- the signs of their activities were everywhere.

At the National Air and Space Museum, visitors were funneled through a single door on each side of the museum and purses and packages were carefully checked.

Security personnel could be seen roaming the roof of the White House with binoculars, and extra guards were placed on the buses that carried legislators to the airport to travel to the funeral of Rep. Joseph P. Addabbo of New York.

At the State Department, guards scrutinized employes more carefully than usual before lowering hydraulically operated steel wedges designed to stop vehicles from crashing through gates to the garage, according to an employe.

However, there was a sense of safety at the State Department, the employe said.

"There are people here who've had embassies overseas burned out from under them," she said. "People don't feel as threatened in this building."

The residents of Quantico, which shares a common border with the Marine base there, were coping with a new system requiring special tags and identification to get past a checkpoint.

"It used to be that town folks could breeze right through the checkpoint with a wave at the guard," said Mayor Lively C. Abel. "Those days are gone forever."