Washington, April 16 (IANS): Two explosions rocked the finish line at the Boston Marathon Monday, injuring at least half a dozen people, CNN reported.

The blasts occurred a few seconds apart on opposite sides of the street.

The first left "a big plume of smoke" hovering over the area, around Boston's Copley Square, the second occurred around 10 seconds later, said a producer for CNN.

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.

The race was halted after the explosions as was subway service into the area.

Paramedics treated several victims at the scene. Troops from the Massachusetts National Guard were assisting police.

An onlooker, Josh Matthews, said he heard the blast, then saw police running towards 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 told CNN.

A spokeswoman for Massachusetts General Hospital said four patients were being treated there.