Los Angeles Dodgers starting pitcher Hyun-Jin Ryu, left, of South Korea, walks by a police officer during batting practice before their baseball game against the San Diego Padres, Monday, April 15, 2013, in Los Angeles. (AP Photo/Mark J. Terrill)

The deadly explosions at the Boston Marathon reverberated on both sides of the Atlantic as cities from Los Angeles to London saw a surge in security.

The White House, New York's Times Square, and the preparations for former Prime Minister Margaret Thatcher's funeral in London all had enhanced and intensified law enforcement presence after Monday's two blasts in Boston.

Police in Los Angeles, Washington, D.C., Detroit, San Diego and Las Vegas monitored landmarks, government buildings, transit hubs and sporting events. Law enforcement agencies also urged the public via Twitter and Facebook to report suspicious activity to the police.

British police were also reviewing security plans for Sunday's London Marathon -- the next major international race -- because of the bombs that killed three people and injured more than 140 in Boston.

And the already robust security operation was stepped up for Wednesday's ceremonial funeral for Thatcher. The event at St. Paul's Cathedral, to be attended by Queen Elizabeth II and other dignitaries, calls for a procession through the streets of London, with Thatcher's flag-draped coffin carried on a horse-drawn carriage.

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In New York, authorities deployed so-called critical response teams- highly visible patrol units that move in packs with lights and sirens, along with more than 1,000 counterterrorism officers. Highly trafficked areas like the Empire State building, Rockefeller Center, St. Patrick's Cathedral, the United Nations and the World Trade Center site were being especially monitored.

Mayor Michael Bloomberg said the police department was fully prepared to protect the city.

"Some of the security steps we are taking may be noticeable," Bloomberg said. "And others will not be."

In Washington, Secretary of Homeland Security Janet Napolitano urged the American public "to be vigilant and to listen to directions from state and local officials."

At the White House, the Secret Service expanded its security perimeter after the attacks, shutting down Pennsylvania Avenue and cordoning off the area with yellow police tape. Several Secret Service patrol cars blocked off entry points, although the White House was not on lockdown and tourists and other onlookers were still allowed in the park across the street.

Speaking late Monday from the White House, President Barack Obama declared that those responsible would "feel the full weight of justice," though he urged a nervous nation not to jump to conclusions and pointedly avoided using the words "terror" or "terrorism."

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However, top lawmakers declared the deadly incident an act of terrorism, and a White House official said it was being treated that way.

In California, emergency management officials activated their statewide threat assessment system, which was established after the Sept. 11 attacks.

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.

In New Jersey, authorities raised security statewide, calling in off-duty state police officers and deploying bomb units, aviation crews, tactical teams and search and rescue assets as a precaution.

Transit and port officials in New York and New Jersey were on heightened alert at bridges, tunnels and on rail lines between the two states, as well as on New York City's subway system and commuter rails.

And at the Port of Los Angeles and Long Beach, as well as at three major Los Angeles airports, including Los Angeles International, law enforcement officials were in a "heightened state of vigilance," with increased patrols, authorities said.

Security was also tightened at sports venues nationwide, though most events were held as planned.

The exceptions were in Boston itself, where Monday night's NHL game between the Bruins and Ottawa Senators was postponed and Tuesday's NBA game between the Celtics and Indiana Pacers was canceled.

Officials announced plans for security reviews of upcoming marathons and road races in cities large and small, including this weekend's marathon in Lansing, Mich., Nashville's Country Music Marathon on April 27, next month's Indianapolis 500 Festival Mini-Marathon and the San Francisco Marathon in June.

Race officials for the Illinois Marathon in Champaign and Urbana, Ill., said they were already fielding calls from worried runners and their families and planned to meet Wednesday to discuss more security measures such as bomb-sniffing dogs.

"I took a call from a very irate parent who screamed at me because I won't cancel the race, because I'm putting her daughter at risk," said Jan Seeley, a director for the Illinois Marathon.

Even so, life went on as usual in locations across the country. In Times Square, tourists crowded the sidewalks seemingly unconcerned about any possible heightened risk. One area was being emptied - but for a movie shoot, not for security, as helicopters buzzed overhead.

Shelly Bybee, 42, a teacher visiting from Austin, Texas, said the idea of public safety was more on her mind, and particularly in New York, given the Sept. 11 attacks.

But, "that was on my mind even before I heard about the explosion in Boston," she said. Overall, "I feel like we should continue living and go about our business."

Boston Marathon Bombing Suspect

This photo released Friday, April 19, 2013 by the Federal Bureau of Investigation shows a suspect that officials identified as Dzhokhar Tsarnaev, being sought by police in the Boston Marathon bombings. (AP Photo/Federal Bureau of Investigation)