Pic: AP

Two explosions went off near the finish line of the Boston Marathon on Monday, killing at least two people and injuring dozens more, as runners completed the course and thousands cheered them on. Race organizers described the blasts as bombs.

Authorities reported a third explosion at the John F. Kennedy Library, elsewhere in the city, more than an hour after the blasts. There were no immediate reports of injuries in that explosion. Authorities said that they were investigating and treating it as related, and the police commissioner urged people in Boston to stay inside.

Video from the finish line at the marathon showed people screaming and an enormous cloud of white smoke, and about 20 seconds between the blasts. The explosions were strong enough to cause at least one runner to collapse, and emergency personnel carried bloody spectators away.

A third, undetonated device was found near the finish line, a House Homeland Security Committee official and three law enforcement officials told NBC News.

Police said at least 23 people had been injured, but the count from hospitals was much higher. Massachusetts General Hospital said that it had 19 patients, Tufts Medical Center nine and Brigham and Women’s Hospital 18 to 20 — a total of 46 to 48.

Of the 19 at Mass General, six were severe, hospital officials said. Dr. Alisdair Conn, chief of emergency medicine, characterized the blasts as a military-style bombing.

“This is like a bomb explosion we hear about in the news in Baghdad or Israel,” he told reporters.

Jackie Bruno, a reporter for New England Cable News, said on Twitter that she saw people’s legs blown off.

“Runners were coming in and saw unspeakable horror,” she said.

Race organizers, in a post on Twitter, said: “There were two bombs that exploded near the finish line in today’s Boston Marathon. We are working with law enforcement to determine exactly what happened.”

President Barack Obama was notified, and his administration was in  touch with state and local authorities, a White House official told NBC  News. Obama directed the administration to provide whatever help was  necessary, the official said.

Suspicious packages were found after the blasts at three Boston subway stops, and authorities were investigating. Within an hour of the blasts, New York police deployed extra security to landmarks, and Pennsylvania Avenue in front of the White House was closed to foot traffic. San Francisco put its police on heightened alert.

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 race began 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.

New England Cable News reported that the emergency response came within seconds.

The Boston transit system suspended some service.

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.

Tracy Connor of NBC News contributed to this report. Reuters also contributed to this report.