News of the horrific bomb blasts at the end of the Boston Marathon this evening quickly took over Twitter and other social networks.
The catastrophic blasts, which killed at least two people and injured dozens more, rocked the city's marathon shortly before 3pm local time.
Within minutes, 24-hour news networks were covering the story, and the news was beamed around the world within seconds online.
The hashtag #BostonMarathon quickly became a trending topic on Twitter - with tweeters heading online to find out what happened or simply convey their shock as feelings ran high.
Prayers goes out to those involved/hurt in #BostonMarathon. WTF is wrong with people man. Just sad— LeBron James (@KingJames) April 15, 2013
Some horrific images being posted on Twitter from #BostonMarathon explosions. Those poor people - pray for them.— Piers Morgan (@piersmorgan) April 15, 2013
Just landed in New York to the terrible news. My thoughts are with the people of Boston. Stay strong.— Ricky Gervais (@rickygervais) April 16, 2013
Oh no, what’s happening in Boston, Massachusetts? It sounds appalling …— Stephen Fry (@stephenfry) April 15, 2013
"Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss." —President Obama— Barack Obama (@BarackObama) April 15, 2013
My thoughts and prayers are with everyone in the city of Boston and all of those affected by this terrible incident.— Justin Timberlake(@jtimberlake) April 15, 2013
The #prayforboston hashtag also trended quickly, as did 'JFK' - in reference to the JFK library which was said to have suffered a third bomb blast across the city, although news of a third blast was unconfirmed.
Singer-songwriter Ed Sheeran was one of those who reacted quickly by using this trending topic, in a message which was retweeted over 40,000 times in the space of a couple of hours.
The harrowing image of the first blast on the finishing line appeared around the world within hours thanks to the power of social media.
Similarly, U.S. basketball star LeBron James sent out his condolences to the victims involved in the Boston Marathon explosions.
He tweeted: 'Prayers goes out to those involved/hurt in #BostonMarathon. WTF is wrong with people man. Just sad.'