Nation on high alert as Obama offers federal assistance to city

US President Barack Obama called Boston mayor Tom Menino and Massachusetts governor Deval Patrick to offer federal assistance in responding to explosions that rocked the end of the Boston Marathon.

Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the blasts.

Participants were also seen lying on the ground as the two explosions tore through the finish line, sending smoke and debris soaring into the air.