As the FBI took over co-ordination of a "potential terrorist inquiry" into the dual bombings that killed three and wounded more than 140, Boston police canceled all leave and urged the city's residents to stay indoors and avoid large groups.
Commuters in Boston and surroundings were warned their bags would be searched on Tuesday morning as the city remained on high alert.
Meanwhile, reaction poured in from around the world.
United Nations Secretary-General Ban Ki-moon deplored "this senseless violence… all the more appalling for taking place at an event renowned for bringing people together from around the world in a spirit of sportsmanship and harmony."
In lieu of a "formal" statement, he said, "For now I just wanted to say that my thoughts are with everyone in Boston.”
CBS quoted other world leaders, including Afghan president Hamid Karzai, London Mayor Boris Johnson, Russian President Vladimir Putin, Australian Prime Minister Julia Gillard as well as Rio's Olympics organizers, as expressing condolences and outrage.
Meanwhile, London marathon officials said that while that race through the British capital would go ahead on Sunday, security would be reviewed for the event.
The BBC quoted British Sports Minister Hugh Robertson as saying Tuesday that "The best way for us to react is to push ahead with the marathon — to send a very clear message that we won't be cowered by this sort of behavior."
London Marathon chief executive Nick Bitel, meanwhile, was cited as saying that organizers were reviewing security in a race through downtown London expected to star British double Olympic champion Mo Farah.
Back in Boston, law enforcement officers were searching an apartment in the Boston suburb of Revere early Tuesday, CBS Boston reported.
Boston Police Commissioner Edward Davis said authorities had received "no specific intelligence that anything was going to happen."
The media filmed investigators leaving the Revere an apartment complex at 364 Ocean Avenue early Tuesday carrying brown paper bags, plastic trash bags and a duffel bag.