Two explosions struck the Boston Marathon on Monday as runners crossed the finish line, injuring at least four people with multiple injuries being reported from the scene.

Police reported at least one explosion and witnesses said there were two, which hit as spectators were cheering on people finishing the Boston Marathon. The first runners in the marathon crossed the finish line around noon.

JFK Library Bombing? 3rd Bomb? Explosions Confirmed as Unrelated to Library 'Incident'

Police, fire and EMS (emergency medical services) were on the scene but there was no indication of how many people are injured, a spokeswoman for the Boston Police Department said. At least three people are dead and dozens injured - including up to 10 with amputated limbs, according to Fox.

"Somebody's leg flew by my head," a spectator, who gave his name as John Ross, told the Boston Herald. "I gave my belt to stop the blood."

"I saw two explosions. The first one was beyond the finish line. I heard a loud bang and I saw smoke rising," Boston Herald reporter Chris Cassidy, who was running in the marathon, told the newspaper. "I kept running and I heard behind me a loud bang. It looked like it was in a trash can or something...There are people who have been hit with debris, people with bloody foreheads."

CLICK HERE FOR LIVE STREAMING COVERAGE AT THE SCENE