Steve Silva, a photographer for The Boston Globe, described "injuries nothing short of horrific." Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people's legs blown off.
Federal officials told NBC News that Boston police were guarding a "possible suspect" who had been wounded in the blasts, but they cautioned that there was no information at the federal level to consider that person a suspect.
A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News. Authorities also reported an explosion at the John F. Kennedy presidential library, elsewhere in the city, more than an hour after the blasts, but police said that it appeared to be caused by a fire. The police commissioner urged people in Boston to stay inside.
Five hospitals reported at least 86 people injured, including at least two children. Dr. Alisdair Conn, chief of emergency medicine at Massachusetts General Hospital, characterized the injuries as like what Americans see on the news from a military-style bombing in Iraq or Israel.
"We still do not know who did this or why, and people shouldn't jump to conclusions before we have all the facts, but make no mistake, we will get to the bottom of this," President Barack Obama said from the White House. "Any responsible individuals, any responsible groups will feel the full weight of justice."
He pledged the full help of the federal government and said: "The American people will say a prayer for Boston tonight."
Suspicious packages were found after the blasts at three Boston subway stops, and authorities were investigating. New York police deployed extra security to landmarks, Pennsylvania Avenue in front of the White House was closed to foot traffic, and the Pentagon tightened security. Federal authorities briefly grounded flights at the Boston airport as a precaution.
The race is a signature event in Boston and has been run since 1897 on Patriots Day, the third Monday in April. Tens of thousands of spectators turn out each year to watch.
Race organizers said that almost 27,000 runners competed, representing 96 countries. The winners were Lelisa Desisa of Ethiopia for the men and Rita Jeptoo of Kenya for the women. A special marker at the 26th mile of the course, yards from the finish, had been set up to honor the 26 dead in the Newtown, Conn., elementary school shooting last December.
The elite men began running at 10 a.m., and the explosions were reported just before 3 p.m. The winners had long ago completed the race - Desisa finished with a time of just over 2 hours, 10 minutes - but the explosions came as masses of other runners were approaching the finish. NBC affiliate WHDH said that storefront windows nearby were blown out.
"Right now I'm in my condo with about 50-60 people I picked up off the street including marathon runners. Setting up a camp," Corey Griffin told NBC News. "They have nowhere to go because everything is shut down. Officials said to get inside. This is crazy."
Evan Kohlmann, a terrorism analyst for NBC News, said that authorities would probably examine residue from the blasts to determine their type.
Adding that it was premature to identify a culprit, he said: "If this was a deliberate act, unfortunately it certainly would reflect something that we're seeing. There's an emphasis on these soft targeted attacks now. We're moving away from the spectacular attacks and we're moving into the small grade, homegrown attacks."
Will Ritter, the spokesman for Massachusetts Senate candidate Gabriel Gomez, who was running the race, told NBC News that he heard what sounded like two explosions and saw smoke rising near the Boston Public Library. He said that he saw three fire engines and police running to the site.
"We heard two really large explosions in rapid succession, about a second apart from each other," Ritter said. "Everybody kind of ducked and hit the ground."
Another witness told WHDH that it sounded like cannon fire.
The AP reported that runners and race organizers were crying as they fled the scene, and that bloody spectators were carried to medical tents intended for exhausted runners. Runners who were still on the 26.2-mile course were being stopped and directed elsewhere, the Massachusetts Emergency Management Agency said. The agency suggested that people trying to reach loved ones use text messaging because of crowded phone lines.
Authorities gave a phone number for people in search of loved ones - 617-635-4520. They encouraged people with information about the blasts to call 1-800-494-TIPS.