The digital clock over the finish line read 4:09:44 when two explosions in the historic heart of Boston turned the 117th running of the city's iconic marathon into a horrific scene of carnage.

Still images and video footage posted on the Internet by journalists and onlookers depicted in vivid detail how the blasts killed at least two people, injured dozens more and sowed terror on a sunny spring afternoon.

The first blast -- on Patriots' Day, a state holiday in Massachusetts -- occurred two hours after Ethiopia's Lelisa Desisa broke through the blue ribbon at Copley Square to become this year's fastest male runner.

One of the first video clips on Twitter -- a screen shot from a local TV station -- depicted the initial blast going off a few meters back from the curb on Boylston Street, behind crowd control barricades and a row of national flags.

So strong was the blast that one runner dropped to their knees as race-day volunteers lining the route put their hands over their ears. A section of barricade toppled over; a Polish flag dipped briefly but sprung back upright.

A second video, posted on the Boston Globe website, captured a thick cloud of dust, onlookers' shouts and screams, a growing cacophony of sirens and, 12 seconds after the first explosion, the second blast a block away.

"Oh my God, oh my God," said the photographer as he turned his camera to the dazed victims sitting on the pavement, clearly in agony, amid a tangle of fencing, in front of a shop with an "All in for Boston" sign in the window.

Next door, windows on the lower floors of One Exeter Plaza -- built in the 1980s and once popularly known among Bostonians as "the Darth Vader Building" -- could be seen shattered by the force of the blast.

A third Internet video, surreally, captured an untethered yellow helium-filled balloon rising peacefully above the chaos.

A still image of the initial blast site showed civilians comforting the injured on a blood-stained pavement while police officers wearing yellow reflective jackets looked on, as if unsure quite what to do.

Mostly empty were spectator stands erected on the other side of Boylston Street, which is lined with hotels, bars, shops, a department store and the main branch of the Boston public library, which was closed for the holiday.

Hayden Cardy, 18, a student from San Antonio, Texas whose mother was among the 27,000 marathon runners, said the first explosion went off about 100 meters from where he stood with his sister.

"There was a very loud boom or explosion ... and from where we were everybody wasn't really sure what had happened. There were so many people, we couldn't see what was going on. We just saw a huge cloud of smoke," he said.

"Then police officers started sprinting to the scene, saying 'Make a hole! Get out of the way!' There was so much noise and confusion." (AFP)