Police in New York, Chicago, Los Angeles and other major urban centers were increasing personnel as a precaution.
Two bombs exploded near the finish line of the Boston Marathon on Monday, killing three people and injuring at least 140 in a terrifying scene of broken glass, smoke and severed limbs, authorities said. A senior U.S. intelligence official said the two other explosive devices found nearby were dismantled.
Homeland Security Secretary Janet Napolitano is directing her agency to provide "whatever assistance" necessary.
President Barack Obama said government "will increase security around the United States as necessary."
"I've directed the full resources of the federal government to help state and local authorities to protect our people, increase security around the United States as necessary, and investigate what happened," the president said in an address to the nation Monday evening. "We will find out who did this and we will hold them accountable."
Authorities in Washington said there was no immediate claim of responsibility.
"We have no information at this time that would indicate the events in Boston might have any connection to the Los Angeles area," said Los Angeles Police Officer Bruce Borihanh Monday afternoon.
The Los Angeles County Sheriff's Department opened an emergency operations center and ordered an increase in patrols at transit areas and other critical locations, including at Dodger Stadium for Monday night's baseball game. The Dodgers and the San Diego Padres met for a 7 p.m. game.
"I have the parks, I have all the county buildings, I have the colleges," said L.A. County Sheriff's Captain Eric Parra. "And so what we've done at the parks is we're increasing patrols. We're looking for the large events. The colleges, we're significantly increasing our patrols, and we're doing that at county buildings for large events."
Security was stepped up at Metro facilities in the Southland, including at Union Station, with additional officers and K-9 units, according to Marc Littman, a spokesman for the transportation agency. Littman said there was no disruption to any service, but passengers will see more officers on patrol. As usual, Littman urged passengers to report any suspicious activities on buses and trains or at stations.
"While the cause of the explosions this morning at the Boston Marathon are still under investigation, our vigilance has been raised," Los Angeles County Sheriff Lee Baca said. "The heightened alert of the Los Angeles County Sheriff's Department includes increased patrols to create a more visible presence where people congregate, such as government buildings, shopping centers, athletics events and public transit."
Baca said the department was working closely with other public safety agencies. Officials said the Los Angeles County Emergency Operations Center had not been fully activated. Baca said the the LASD had opened its internal Department Operations Center.
The LAPD did activate its own operations center.
"We are working closely with our law enforcement partners to monitor the situation and stand ready to aid Boston in any way possible," said Los Angeles Mayor Antonio Villaraigosa. "The LAPD and other public safety agencies will remain vigilant at all public and sporting events in Los Angeles."
Travelers will also see stepped-up security at Los Angeles International Airport, and officials at the Port of Los Angeles were under a heightened state of alert. Behind-the-scenes security procedures were also deployed.
Los Angeles World Airports released a statement Monday: "Los Angeles Airport Police continues to monitor the tragic explosions that occurred earlier today at Copley Square in Boston. While there is no information to suggest that there is any nexus to the Los Angeles area, in an abundance of caution, Airport Police has increased its presence at LAX, LA/Ontario International, and Van Nuys airports by deploying bomb-detection canines, additional patrol officers and security personnel."
Riders on the MTA bus or rail system can take the following actions to alert authorities to suspicious activity: Alert deputies or Metro personnel; Press the emergency intercom in the stations or on the trains; Inform bus operators; Call the L.A. County Sheriff's Dept. at (323) 563-5000.
In Orange County, Sheriff Sandra Hutchens said deputies would "have an increased presence at local sporting/entertainment venues and other large gathering places."
The Federal Aviation Administration established a no-fly zone over the site of two Boston explosions. The agency said in a notice issued Monday about an hour after the explosions that a no-fly zone with a 3.5-mile radius has been created over 811 Boylston Street. That was later reduced to a 2-mile radius. The zone was limited to flights under 3,000 feet in altitude, which is lower than most airliners would fly except when taking off or landing.
Cellphone service was temporarily shut down in the Boston area to prevent any potential remote detonations of explosives, according to a law enforcement official.
A government official told ABC News that security has been beefed up at Boston area airports and additional scrutiny is being given to outbound passengers heading to international destinations.
New York police deployed critical response teams around the city. "The NYPD has stepped up security at strategic locations and critical infrastructure, including our subways. Some of the security steps we are taking may be noticeable ... and others will not be," New York Mayor Michael Bloomberg said in a statement.
Transportation officials at airports and train stations around the country also reported stepped up security. In Washington, transit officials said in a Twitter message they knew of no specific or credible threat to the city's subway system, but "extra officers are on duty for increased security" anyway. It was the same case in New Jersey, where the state's transit agency said it was in operating "a state of heightened alert."
The California Highway Patrol said in a statement: "We have an effective process for handling any threats when discovered and we are continuing to work with our federal, state, and local law enforcement partners to ensure the safety of the people of California. We urge the public to report to law enforcement anything that appears out of the ordinary."
California emergency management officials activated their statewide threat assessment system, which was established following the Sept. 11, 2001 World Trade Center bombings.
Officials in San Francisco, Los Angeles, Orange County, San Diego and Sacramento were reviewing information from federal authorities for possible threats, said Kelly Huston, assistant secretary of the California Emergency Management System.
"The Anaheim Police Department and Anaheim Fire and Rescue remain diligent in monitoring activity in Boston and any nexus to our community. We are in contact with Federal agencies as well as monitoring local intelligence activity," an Anaheim spokesperson said in a statement.
Palm Springs International Airport Executive Director Tom Nolan declined to discuss airport security specifics in light of what occurred in Boston, other than to say that "any time there is an incident nationally or within a region, we always take that very seriously."
Administrators at San Diego International Airport made no immediate changes, since the Transportation Security Administration had not directed them to do so, according to airport spokeswoman Rebecca Bloomfield.
San Francisco police Officer Albie Esparza says patrol officers are being asked to be extra vigilant around critical infrastructure and areas where large crowds gather. The department is also going to rethink security for the upcoming San Francisco Marathon in June and the Bay to Breakers foot race in May.
San Jose police say the department is not modifying its field deployment of officers or security at any critical infrastructure sites, like the airport.
Police in Washington, San Diego, Vegas and Atlanta were monitoring events closely and assessing potential increases in security measures. Agencies were also stepping up social media response, telling the public via Twitter and Facebook to report suspicious activity to the police.
At Dulles and National Airports in Washington, D.C., the airport authority increased police patrols and patrols by K-9 units.
In Dallas, airport media officials said they are not taking additional security measures at this time.
In Seattle, police increased patrols in neighborhoods and around government buildings and other facilities. In Colorado a statewide alert was sent out advising law enforcement agencies to look out for suspicious activities.
The Secret Service expanded its security perimeter at the White House on Monday following the explosions at the Boston Marathon. Secret Service spokesman Ed Donovan said the measure was taken "out of an abundance of caution" and noted that it was not unusual to expand or contract the security perimeters.
The White House was not on lockdown and tourists and other onlookers were still able to be in the park across the street from the executive mansion.
A Defense Department official said there were no increases to force protection levels at military bases in the U.S. as a result of Boston bombings.
Police in Britain were reviewing security plans for the upcoming London Marathon scheduled for Sunday. The London Marathon is the next major international marathon. The London Marathon is a hugely popular race. Last year, some 37,500 athletes competed, with many more watching the springtime event. London has long been considered a top target for international terrorists, with the government saying the threat level is "substantial." In 2005, a series of suicide attacks on the public transport system in the British capital killed 52 people.
London police already were preparing a major security operation for Wednesday's ceremonial funeral for former Prime Minister Margaret Thatcher, an event at St. Paul's Cathedral that will be attended by Queen Elizabeth II and many dignitaries.
Boston Police meanwhile urged citizens to stay indoors and not to congregate in large groups Monday while the investigation continued.
The only nuclear power plant in Massachusetts increased security Monday. The Pilgrim Nuclear Power Station in Plymouth, Mass., about 45 miles south of Boston, was already shut down for refueling when the explosions occurred.
The Associated Press and City News Service contributed to this report.