Local, state and national authorities scrambled to respond to security threats after two bombs detonated Monday afternoon near the finish line of the Boston Marathon, killing three and injuring at least 140 runners and spectators.

Among the dead include an 8-year-old boy, news outlets have reported. At least 17 of the injured were in critical condition and at least 10 suffered from amputated limbs, according to officials. There have been no reports of injuries to Yale students or staff members from the two explosions, which occurred nearly simultaneously around 3 p.m. as runners finished the race and thousands looked on.

Police also found several suspicious packages, an unconfirmed number of which proved to be harmless, located in public places throughout Boston, including in three hotels and neighboring suburbs. The Boston police commissioner issued a statement Monday afternoon urging people to stay indoors and not to congregate in large groups, and police in cities as far away as New York City announced they would step up security in the wake of the tragedy.

Those in the vicinity of the blast described the experience as terrifying. Ramana Gorrepati, a doctoral student at Cornell, witnessed the explosions from across the street on the second floor of Lord & Taylor.

“You hear these two earth-shattering explosions, and the glass that I was just looking through just shattered,” Gorrepati said.

Gorrepati said he saw six runners down at the scene, some of whom had lost limbs. He said emergency personnel urged onlookers to back away from the scene as they recovered injured individuals, adding that officials found suspicious devices at the nearby Mandarin Oriental hotel, located on Boylston Street.

Boston University junior Daliena St. Germain said that her building was put on lockdown after the bombs exploded about a block and a half away. St. Germain said she had been watching the race at the finish line, the exact location that the bomb detonated, but decided to leave before the end of the race. Some of her classmates were in tears, she said, as they called their loved ones following the explosions.

“I’m still kind of shaky. I thought it would wear off by now,” St. Germain said hours after the explosion.

In New Haven, the Yale and New Haven police departments sent officers, detectives and bomb-sniffing dogs to Ingalls Rink on Monday during the hockey team’s victory celebration to ensure safety, although authorities did not expect anything similar to occur on campus, according to the New Haven Independent.

Authorities have yet to name suspects or a motive for the bombing publicly, which the White House called an act of terror. Nevertheless, President Obama, in brief statement made from the White House early Monday evening, emphasized that the guilty would be hunted and brought to justice.

Several Yale students, including Clare Monfredo ’13 and Calvin Bohn ’14, attended the Boston Marathon, although neither was injured. Yale professor Ray Fair was scheduled to run in the marathon and said he likely would have finished at about the time at which the bombs detonated, but withdrew because of an injury.

Some buildings in Boston were evacuated after the explosions, while others were put on lockdown. Tufts University briefly evacuated its medical center because of a suspicious package, and Harvard did the same for the Kennedy School. The Brookline Police robocalled residents urging them to stay out of Beacon Street and evacuated Washington Square and Coolidge Corner.

This year’s marathon, which drew about 27,000 runners, honored the 26 victims who died in the Sandy Hook school shooting with a special marker at the end of mile 26, as well as with 26 seconds of silence before the marathon began.

Michelle Hackman and Matthew Lloyd-Thomas contributed reporting.