Boston (CNN) -- At least two explosions rocked the finish line at the Boston Marathon on Monday afternoon, injuring at least a half-dozen people, a CNN producer at the scene said.
The blasts occurred a few seconds apart on opposite sides of the street, CNN Producer Matt Frucci reported. The first left "a big plume of smoke" hovering over the area, around Boston's Copley Square; the second occurred perhaps 10 seconds later, he said.
The explosions occurred about 2:45 p.m., about an hour after the first of the race's nearly 27,000 runners had crossed the finish line, Frucci said. The race was halted after the explosions as was subway service into the area.
Paramedics were treating several victims at the scene, and police ordered onlookers to back away from the area. Troops from the Massachusetts National Guard were assisting police as well.
Onlooker Josh Matthews said he heard the blast, then saw police running toward the scene.
"We just heard a lot of sirens, and people were kind of frantic, and it was a bad situation, so we got out of there," he said.
A spokeswoman for Massachusetts General Hospital said four patients were being treated there.
