Here are 10 important points to note:
Several big US cities including New York, Los Angeles, Chicago and Washington have been put on alert. (Read)
Cellphone service has been shut down in the Boston area to prevent any potential remote detonation of explosives. (Read) While two bombs exploded at the finish line of the race, Boston police said at least two more devices nearby, according to reports.
Flight movement over the bomb blast site in Boston has been banned. (Read)
Federal investigating agencies and law enforcement bodies have increased their check on all outgoing international flights, according to reports.
India has asked its nationals to contact the embassy if they were affected in the blasts. "Any Indian nationals if affected by tragic blasts at #BostonMarathon may please contact @IndianEmbassyUS /202-939-7000," Indian ambassador to the US Nirupama Rao has tweeted.
The Secret Service has expanded its security perimeter at the White House following the explosions Boston. It has shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. (Read)
The White House however is not on lockdown and tourists and other onlookers are still able to be in the park across the street from the executive mansion.
In Boston, security at hotels and other prominent locations has been stepped up. Officials have closed down a 15-block radius around the blast site, shut some transit stops.
Landings were briefly halted at Boston Logan International Airport and the Boston Symphony Orchestra cancelled its Monday night concert. A Boston Celtics game scheduled for Tuesday was also cancelled.
The London Marathon will be held as scheduled on Sunday, organisers said. The police, however, said they would review security plans. (Read)