Expect to see more people in uniform, a greater display of law enforcement firepower, added security in places where it had been relaxed and increased efforts to poke into backpacks and purses.

A day after an attack on an unlikely Boston target, hardly any place in Washington felt safe from aggression. It was a day when the numbing routine of submitting to scrutiny took on a comforting urgency.

Watchful armed guards were out in great numbers Tuesday at all of the places long perceived as possible targets: airports, Metro, monuments, museums and the various institutions of governance. People who run the area’s big arenas and stadiums said they will revisit their precautions. Those who bear the security burden for schools, malls, parking garages and office buildings said they were rethinking, reviewing and consulting with their peers.

“Washington is an icon,” said Rafi Ron, a former top Israeli security official who now runs a Dulles-based counterterrorism consulting firm. “There is no way we can protect all the soft targets. Once you manage to protect the high-value targets, then the second in line become the high-value targets. We need to understand that this is going to happen from time to time as long as there are people out there who are motivated to do this.”

What Monday’s attack on Boston will mean for Washington in the long term was hard to say. “Beefing up” was the catch phrase for a day when few brand-new steps could be taken. In a city where big events and mass gatherings are a matter of routine, and that has more metal detectors, bomb sniffing dogs and security forces per capita than just about any other place, is there anything more to be done?

“Public attitude is probably the most important thing,” Ron said. “We’ve been hearing the slogans of ‘When you see something, say something.’ Nobody takes those seriously because nothing had happened for a rather long time. Either they’re not paying attention or, worst case, even when they see something that is disturbing, they wait for somebody else to report it.”

The reality that any large gathering could become a target was evident Tuesday to those who turned out for the District’s annual Emancipation Day parade.

Groups of police were on nearly every corner along Pennsylvania Avenue all the way to the front of the U.S. Capitol. They came in a wide variety of uniforms, including a few U.S. Park Police officers in olive-green camouflage.

D.C. Mayor Vincent C. Gray (D) said there was no thought of canceling the parade.

“We are a resilient city,” the mayor said. “We are a resilient nation. What happened in Boston is a tragedy. [But] it’s not going to run us off.”

Tom and Celeste Letchworth, visiting from Marion, Ark., had no idea that Tuesday was Emancipation Day. They ventured down to Pennsylvania Avenue, map in hand, to take in a bit of the city’s history. They were disappointed they couldn’t get closer to the White House, but said they understood the increased security.

“I’m sure we got into a bit of a lull after 9/11,” Tom Letchworth said. “It means we have to be extra vigilant.”

Metro said it had extra K9 units and police at rail stations Tuesday.

“There is an expectation from our customers and our employees,” Metro Transit Police Chief Ron Pavlik said. “We want to let them know we’re out there and we’re taking this seriously. Until we know more intelligence from Boston, we’re doing this out of an abundance of caution.”

Metro spokesman Dan Stessel said riders should not expect “an announcement when we step down from the current level” of added security.

With July 4 less than three months away, Pavlik said law enforcement agencies and federal security officials will review security in handling large crowds.

“We’re going to make decisions based on the intelligence we have,” he said.

The transit agency is installing new security cameras at stations that will have the ability to record audio as well as video.

Sgt. Paul Brooks, spokesman for the U.S. Park Police, which patrols the Mall and handles many large public gatherings there, declined to give details about any extra steps the department might be taking.

“You can rest assured that we’re out there and maintaining diligent patrols along our areas of responsibility to include the monuments [and] memorials,” he said.

Officials who provide security for major events said they will enhance security for upcoming events, including the Verizon Center, which already searches bags and sweeps patrons with a metal-detecting wand.

In Richmond, Police Maj. Steve Drew said precautions will be taken at a NASCAR race coming up at the 94,000-seat Richmond International Speedway.

“We will learn from what happened in Boston,” Drew said. “I don’t think that there’s a law enforcement agency around this country that isn’t on a heightened awareness or heightened alert, reviewing . . . manpower, strategies [and] planning before big events.”

Baltimore police dispatched a team to Boston. “God forbid we have a similar occurrence locally, we want to make sure that we learn best practices that are occurring up there,” said Anthony Guglielmi, chief spokesman for the Baltimore police.

He said extra officers were being sent to transportation hubs and to Camden Yards, where the Orioles start a nine-game home stand Tuesday. “We’re going to be looking at any type of gathering that would attract a large group of people,” he said.

Baltimore police also are reviewing all upcoming special events, including a foot race this weekend, the Preakness horse race May 18 and the Baltimore Ten-Miler on June 15.

“There’s no credible threats to Baltimore,” Guglielmi said. “This is just some pro-active planning. This is in response to what’s happening up in Boston. This is to put people at ease. And it’s really to remind everyone . . . if you see something, say something.”

After Monday’s explosions, Ron went through the available videos frame by frame in stop action.

“Nobody really paid attention to what [the bomber] was doing, and that’s quite alarming,” he said. “I’m confident that the placement of those devices was done under the eyes of many people, possibly also under the eyes of the security personnel. When you look at most of the first responders prior to the explosions, they were all standing there and facing the runners.”

Mark Berman, Dana Hedgpeth, Peter Hermann and Rachel S. Karas contributed to this report.