US police departments from New York to Los Angeles boosted security and were on heightened alert on Monday after a double blast killed at least two people and injured 23 others at the Boston marathon.
While there is yet no official word on whether the explosions were an act of terrorism, New York and other cities quickly took extra precautions.
New York mayor Michael Bloomberg said security was being stepped up at "strategic locations and critical infrastructure, including our subways."
"We have 1,000 members of the NYPD assigned to counterterrorism duties, and they - along with the entire NYPD and the investments we have made in counterterrorism infrastructure - are being fully mobilised to protect our city," Bloomberg said in a statement.
"As law enforcement authorities investigate today's explosions in Boston, I ask all New Yorkers to keep the victims and their families in your thoughts and prayers."
Police in the nation's capital of Washington, DC - which like New York was a target of the September 11, 2001 terrorist attacks - have also "heightened security," spokeswoman Saray Leon told AFP.
Los Angeles police chief Charlie Beck announced an increased police presence at "all scheduled sports events in the near future," beginning with the Dodgers-San Diego Padres baseball game on Monday evening.
The additional police presence would include deployment of bomb-squad personnel, dogs and other "precautions geared to preventing a similar event."
Security was also stepped up at train stations, government buildings, shopping centers and "places where people congregate," Los Angeles County Sheriff Lee Baca said.
Police in the West Coast city of San Francisco said they were on "heightened alert" and urged people to call 911 if they see anything suspicious "in light of the Boston marathon explosions."