Throughout the city, large numbers of heavily armed police forces and SWAT team members are expected to be out in force.

Amidst the chaos Monday, it was hard for many runners and spectators to discern the origin of the two loud noises, spaced about 12 seconds apart.

Marathon attendee Wendy Lehman, who was awaiting her sister’s arrival at the finish line, said that she first thought the sounds might have been a speaker exploding or a signal that the race was ending.

“Not even a minute later, the second one went off, and we saw people falling into the street,” Lehman said. “The bomb threw them—blasted them back. And we started running in the other direction.”

In the aftermath of the explosions, marathon participants said they had trouble communicating with loved ones and finding transportation away from the scene.

“The biggest problem for a lot of runners right now is getting through the downtown area,” said marathon runner Malcolm Pain. “Subways are closing down, so you know it’s just havoc.”

Following the consecutive explosions, heavy use of mobile devices in the downtown Boston area overwhelmed cell phone service, preventing many users from making calls.

People at the scene of the explosions lauded the service provided by the hundreds of first responders, saying that their work was orderly and effective.

Gary Varano, who ran in the race with his wife, described the response as “instantaneous.”

“It was organized as it can be,” he said.

The nearly 13,000 medical personnel, many of whom were volunteers, came to the race expecting to treat fatigue and dehydration, but instead found themselves attending to runners and spectators with serious, even life-threatening, injuries.

Pierre A. d’Hemecourt, a doctor at Boston Children’s Hospital, was staffing the medical tent near the race’s finish line at the time of the blasts. Around two hours later, he walked away from the scene with blood on his clothes and tears on his face.

“I knew that, immediately, I had to reorganize the tent,” he said. “The medical tent was used as a mass unit to triage and get people initially stabilized and off to the hospital.”

After the smoke cleared on Boylston St., an outpouring of support rippled across the country. In Washington D.C., Speaker of the House of Representatives John Boehner held a moment of silence, and a somber President Barack Obama addressed the nation from the White House. His short speech, which he delivered from the press room just after 6 p.m., expressed sympathy for the city of Boston and vowed to identify and hold accountable those who were responsible for the attack.

“The American people will say a prayer for Boston tonight,” Obama said.

—Staff writer Elizabeth S. Auritt contributed to the reporting of this story.

—Staff writer Pooja Podugu can be reached at podugu@college.harvard.edu. Follow her on Twitter @PoojaPodugu.

—Staff writer Samuel Y. Weinstock can be reached at sweinstock@college.harvard.edu. Follow him on Twitter @syweinstock.

—Staff writer Zohra D. Yaqhubi can be reached at zyaqhubi@college.harvard.edu. Follow her on Twitter at @zohradyaqhubi.