Mailboxes, newspaper vending machines and anything else that could hold explosives may be unbolted from streets and hauled away.

And Porta Potties -- an icon of comfort and convenience at large outdoor gatherings for generations -- may disappear as police and other law enforcement officials rethink the way they protect crowds at public events following the deadly bombings at the Boston Marathon last week.

Some security upgrades under consideration would go mostly unnoticed, including the drones overhead and, underfoot, the manholes that would be bolted closed.

But some would be painfully obvious, including the hours-long lines at gates and metal detectors in places where they never used to be, and the rows of portable toilets that would go missing after decades of providing reliable relief to the masses.

"You have to leave or wear a diaper -- and some people do," said Lt. John Grimpel, a spokesman for the New York City Police Department, referring to the stark choice tens of thousands of New Year's Eve revelers have faced since Dec. 31, 2001, four months after the 9/11 attacks, when portable toilets were removed for good from the celebration in Times Square.

The pledge President Obama delivered at a service for the victims of the marathon bombings that Boston "will run again" was an affirming message for a wounded city. But for the people whose job it is to secure the marathons, parades, sporting events, rallies and similar outdoor events around the country, the president's pledge was as much of a back-to-the-drawing-board work order as a soothing consolation.

Emerging technology is changing the domestic war on terror on both sides: terrorists now detonate bombs remotely by cell phone. The recent profusion of surveillance cameras, and soon possibly drones, aid in their identity and capture.

Boston and state police did not return phone calls Friday seeking to learn how they might upgrade security at next year's marathon and similar outdoor events. Spokesmen said they were focused on the massive manhunt for the last suspect still at-large, which ended under the shrinkwrap of a boat parked in a Watertown backyard late that night.

In Lawrence, Police Chief John Romero said fully securing many outdoor places where large numbers of people concentrate is impossible given the logistics, the delays that would occur by screening everyone and the ingrained cultural expectation in the United States that living in a free country means free access.

The biggest challenge may be the wide-open subway systems in major municipalities that admit millions of people daily with just the swipe of a card, Romero said. In New York, where Romero was a precinct commander and deputy inspector before coming to Lawrence, subways provide 5.4 million rides every workday.

In Boston, when the MBTA was shut for most of the day on Friday in an historic first for the region as the search for a suspect in the marathon bombing intensified, local riders got an idea of what happens when terror disrupts a far-flung urban mass transit system.

"There's no way you can efficiently search everyone coming through," Romero said about transit systems. "The impact on rush hour would be (unacceptable). You can make periodic stops, but you're not going to be able to check every person. You're limited in what you can do."

Boston's transit system is not nearly so sprawling or heavily traveled as the one Romero helped protect in New York, and its schedule of prominent outdoor public events is much less crammed. But from the marathon in April to the fireworks on the Common in July to the Head of the Charles regatta in November, few weekends pass in Boston without a major event, many with international allure.

Today's events include the 31st Annual Run of the Charles Canoe and Kayak Race on the Charles River through Boston and 10 other municipalities, which last year attracted 1,500 paddlers and thousands more spectators. Police from five of the 11 municipalities that the course runs through will provide security on the riverbanks and a state police marine unit will have two boats on the Charles, said event director Meg Schermerhorn.

"Of course I thought about it, a lot," Schermerhorn said about whether she reconsidered security at the race following the marathon bombings, then concluded she was satisfied.

"We go by what they think we need," she said, referring to local and state police. "We have a lot of police on the water and at the finish line. We have volunteers along the course. We have ham operators, who also were at the marathon."

In the Merrimack Valley, one of the biggest annual outdoor events is the Feaster Five, a five-mile road race first run on Thanksgiving Day in Andover 25 years ago, which last year attracted 10,720 runners. Bill Pennington organized the races for the first five years and now organizes the Run For The Troops, a race that raises money to provide housing for disabled veterans. The race attracted 2,000 runners when it was last run on April 3.

Pennington said he worries about a terror trickle-down, where smaller events like his could become easier targets because they provide less security. At the same time, he said he worries that not much more can be done to thwart terror at outdoor public events.

"There are thousands and thousands of road races and events throughout the country," said Pennington, who made it to mile 25 at the Boston Marathon on Monday before he and his daughter were forced off the course. "I don't think there's a hell of a lot you can do. If you protect the finish line, then at mile two-and-a-half somebody could do something crazy. There were police at the Boston Marathon on every block. Every intersection, there was a policeman or National Guardsman. And still it happened."

The obstacles to enhancing security at public events go beyond the expense, logistics and practicalities, and include more than foiling terrorists.

A decade ago, the surveillance cameras that are now commonplace at street corners, bank lobbies and delicatessens -- and that provided the breakthrough in the search for suspects in the marathon bombing -- were fiercely opposed by civil libertarians, who said they violated privacy.

Grimpel, the NYPD lieutenant, said he expects there will be similar opposition if police departments begin launching drones to monitor crowds at outdoor events. But he said the department took a small step in that direction last year when it deployed four 20-foot retractable watchtowers, each equipped with infrared cameras and mounted on vehicles, to monitor high-crime areas and prevent looting in neighborhoods hardest hit by Hurricane Sandy.

"Surveillance, whether it's with a camera or drone, doesn't stop crime from taking place," said Carol Rose, executive director of the American Civil Liberties Union of Massachusetts. "These horrible attacks (at the marathon) indicate that. This was a heavily surveilled area and a heavily policed area. And it didn't prevent these horrible attacks. So it's important that people don't think technology is going to solve all the public security issues when other kinds of investments, including community policing and outreach, may go much further to keep us safe."

Grimpel acknowledged that establishing a balance between security and freedom is a special challenge in countries like the United States.

"In a democratic, open society," he said, "it's next to impossible to be 100 percent secure."