Police in Los Angeles, New York City, Washington, London and other cities around the world stepped up security Monday following deadly explosions at the Boston Marathon that left at least two people dead and many others injured.

In Los Angeles, the Sheriff's Department activated its emergency operations centre and increased patrols at transit hubs, schools and county buildings, while in New York, critical response teams were deployed citywide and officials stepped up security at hotels and other prominent locations. 

California emergency management officials activated their statewide threat assessment system, which was established after the Sept. 11, 2001 attacks. And officials in multiple cities and counties throughout the state were reviewing information from federal authorities for possible threats. 

Meanwhile, police in Washington, San Diego, Las Vegas, Detroit and Atlanta were monitoring events closely and assessing potential increases in security measures. 

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

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The explosions also spurred a review of security at upcoming sporting events including the London Marathon set for next Sunday. (Winslow Townson/Associated Press)

Agencies were also stepping up their social media response, telling the public via Twitter and Facebook to report suspicious activity to the police. 

In Seattle, police increased patrols in neighbourhoods and around government buildings and other facilities. In Colorado a statewide alert was sent out advising law enforcement to look out for suspicious activities. 

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Police at three major Los Angeles area airports, including Los Angeles International, were in a "heightened state of vigilance" with increased patrols, said Chief of Airport Police Patrick Gannon. 

"We have no indications that suggest there's a nexus from Boston to the Los Angeles airport, but in an overabundance of caution, we have heightened our patrols," Gannon said. 

At the Port of Los Angeles and Long Beach, police increased security procedures by doing critical site checks of property around the port complex as well as co-ordinating with multiple local and federal law enforcement officers. 

Security heightened at sporting events

The explosions also spurred a review of security at upcoming sporting events. British police said they were re-examining plans for Sunday's London Marathon — the next major international marathon. 

The San Francisco Police Department was also rethinking security for the upcoming San Francisco Marathon in June and the Bay to Breakers race in May. In Indianapolis, authorities were reviewing security for next month's 500 Festival Mini-Marathon, while in Nashville, increased security precautions were being considered for the Country Music Marathon on April 27. Stepped up security was also put in place for this weekend's marathon in Lansing, Mich. 

Security was heightened for a number of sporting events Monday night, including the Dodgers-Padres baseball game in Los Angeles and the Nationals-Marlins game in Miami. But Major League Baseball said no changes were planned to ceremonies at ballparks around the country to commemorate Jackie Robinson Day, though several teams informed the league they planned moments of silence. 

In Miami, a police officer with a dog patrolled near a ballpark entrance — an unusual sight at the venue — two hours before the game. In Tennessee, hockey fans watching the Nashville Predators play the Vancouver Canucks were seeing "more visible presence around the building," team spokesman Kevin Wilson said.