Two explosions described by race organizers as bombs went off near the finish line of the Boston Marathon on Monday as runners completed the course and thousands cheered them on. At least two people were killed and 23 injured, police said.

Video from the finish line showed screams and an enormous cloud of white smoke, and about 20 seconds between the blasts. The Associated Press reported that bloody spectators were being carried to a medical tent that had been set up to care for tired runners.

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.

A House Homeland Security Committee official told NBC News that Boston firefighters reported discovering an undetonated device at an intersection near the finish.

The injury count from area hospitals exceeded that initially given by the police. Massachusetts General Hospital said that it had 19 patients, and Tufts Medical Center said it had nine.

Serghino Rene, 30, who works in alumni relations, was volunteering near the finish line not long before the bombing but was a few blocks away when he heard the explosion.

“We saw two big puffs. I thought maybe it was fireworks. Then it went off again. And then all of a sudden we heard people crying and running away. It was a huge horde of people just running away,” he said. “We just got away from the scene and away from tall buildings. I’m scared and emotional. I’m wondering if there are people who I was volunteering with who were still there and got hurt.”

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.

Within an hour of the blasts, New York police deployed counterterrorism vehicles 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, but the explosions came as masses of other runners were approaching the finish. NBC affiliate WHDH said that storefront windows nearby were blown out.

Will Ritter, the spokesman for Massachusetts Senate candidate Gabriel Gomez, 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.”

Janet Wu, a reporter for WHDH, told NBC News that she heard two loud explosions. A witness told WHDH that it sounded like cannon fire.

The AP reported that runners and race organizers were crying as they fled the scene. 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.

The Boston police bomb squad was en route. Police, fire and medical technicians were responding. New England Cable News reported that the emergency response came within seconds.

The Boston transit system suspended some service.

OnTheSceneNY Condolences Goes out to those Lost !!!