Three are dead, including an 8-year-old boy and more than 130 injured.
Two bombs exploded in the crowded streets near the finish line of the Boston Marathon early this morning at 2.50am (Singapore time), leaving a bloody scene of shattered glass and severed limbs.
In forceful statement immediately after the incident, President Barack Obama said that "We will find out who did this; we'll find out why they did this. Any responsible individuals, any responsible groups will feel the full weight of justice."
Reports from the US say as many as two unexploded bombs were also found near the end of the 42.165 km course, but they were safely disarmed.
The fiery twin blasts took place about 10 seconds and 90m (100 yards) apart. The impact of the blasts knocked spectators and at least one runner off their feet, shattered windows up to three stories high and sent dense plumes of smoke rising over the street and through the fluttering national flags that lined the course.
One of the devasting images to emerge is of a man in a cowboy hat, 52 year old Carlos Arredondo, who was seen helping a paramedic with a badly wounded man in a wheelchair.
About 23,000 runners took part in this year's race - one of the world's oldest and most prestigious marathons.
The race winds up near Copley Square, not far from the landmark Prudential Center and the Boston Public Library. It is held on Patriots Day, which commemorates the first battles of the American Revolution, at Concord and Lexington in 1775.
According to media reports from the US, the attack may have been timed for maximum carnage as the four-hour mark is typically a crowded time near the finish line because of the slow-but-steady recreational runners completing the race and typically, there are crowds of friends and relatives to cheer them on.
Boston Police Commissioner Ed Davis said at a news briefing that police also detonated a suspicious package found along the race course, but it did not appear to have been an explosive device.
Moments after the incident Google launched an initiative to help people look for loved ones whom they think are affected by the blast. Called "Person Finder", it is currently tracking 4,800 people on the page.
There are no reports of any Singaporean casualties in the blasts.
Singaporeans in Boston who require consular assistance please contact +19172934540for the New York City office or +12022564629 Washington D.C. office.
