At least six people have been injured after two large explosions struck near the finish line of the Boston Marathon on Monday.
Television footage showed scenes of confusion, with streets littered with debris and blood, and paramedics carrying stretchers.
Bloody spectators were being carried to the medical tent that had been set up to care for fatigued runners. Police wove through competitors as they ran back toward the course.
A spokeswoman for Massachusetts General Hospital said four patients were being treated there. Neither race officials nor public officials could immediately estimate the number or degree of injuries.
The explosions occurred not long 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.
A New York City Police Department spokesman said the city has stepped up security around landmarks in Manhattan, including near prominent hotels, in response to the blast.
Boston is a major metropolis located in the northeastern U.S. The marathon is a significant event the city hosts every year.