Some of the reactions in media and social media like Twitter:
"In Boston, explosions transform a scene of celebration into one of tragedy," said the Washington Post in an editorial noting, "In the weeks and months after the 2001 terrorist attacks, every iconic sports event became an occasion for anxiety."
"The simple joy of a 26.2-mile run was shattered on Monday," wrote the New York Times in an editorial titled "Bombs at the Marathon."
Calling a marathon as "the most unifying of sporting events," the daily expressed confidence "the marathon will be back next year, no matter how much security is required, and the crowds should yell twice as loudly."
The Wall Street Journal in a similar vein said "Terror in Boston" was "A reminder of how vulnerable the US homeland still is."
tom allison @ttallison: Boston is a strange target...I wonder if this attack was planned for the NYC Marathon, but Sandy changed their plans.
Glen Coco @fatymae: Does anyone know where we can send out donations to the Boston Marathon people who were severely injured?
Srishti Tomer @CantFindHandle: "Indian Embassy Helpline for Indians affected by the Boston Marathon bombing: 202 939 7000.
CJ Alley @ShelteredRabbit: My dad ran the Boston Marathon twice and he never had to worry about terrorists. It's crazy what kind of awful people live in this world.
MαнoмιeŠнαsα @Mi__ssa: Our hearts and prayers go out to those at the Boston marathon and their loved ones. :("
Messiah Refulfilled @essene: Boston Marathon bombs maybe intended for provoking US(/EU/NATO)to step in bringing down Assad-If so,more arrays of bombs to be anticipated?
ulises estrella @UliAnon: The attack on the Boston marathon should remind us that terrorism isn't just foreign. Its also domestic.
Jeff Dixon10 @jeffdixon_11: The more I think of that explosion at the Boston Marathon, the more mad I get. Why? I just don't understand.
Brad Smith @YahooThurry: There is no spot where God is not - thoughts and prayers to out to all impacted #bostonmarathon.
William Badenhorst @WilliamWealth: Hectic news bout the #BostonMarathon. Prayers and thoughts for the deceased's families and those that got injured.
Big Daddy @Groover4life: Our thoughts and prayers to all those who lost their lives in the Boston Marathon explosions. God save us from terrorism.
BrookLyn @BrookLyn1825: Remember a few months ago politicians and pundits complained abt police and firemen pensions.
Azeem Salehmohamed @ASalehmohamed: Three blasts, if this was done by a Muslim , it would be called Terrorism, not just explosions by news organisations.
Nathan Parsons @NathanParsons98: Initially, hearing about the tragedy at the Boston Marathon today horrified me. Now I'm angry. Bring these cowards to justice.
Eric Billimoria @EBillimoriaCPA: Boston Marathon explosions attract an outpouring of help from city's residents.
Donn Corbin @donncorb10: Heart and prayers go out to all the families in the Boston marathon today. The world can't be this evil man."
Chris Visser @bigchrisvis: Thoughts and prayers go out to those who lost loved ones in the bombing #BostonMarathon To those who were injured, recover soon.
Seanny Mac Parody @seannymacc: It sickens me watching the news about the Boston Marathon bomb. Sending prayers out that way and hope they find the person who caused this.
kaykay.carmen @karmenlunarxisc: Yup people, fake photos and tweets have been circulating on boston marathon. Beware and do check.
Marilyn McKenna @mckennamarilyn: All Americans are marathoners! Wear a race shirt tomorrow to show solidarity with #BostonMarathon victims.
Vancouver Events Gal @catherinebarr: Horrors like the #BostonMarathon make me want to run away and hide, then I remember how important it is to stand up and fight.
Megan Choi @socialleaders12: Time for change. This world is in desperate need of healing.
Natasja McPherson @NatasjaMcPherso: Woke up to the Boston marathon story. so sad. there's some sick people on this planet.Lindsay Lohan @lindsaylohan: Sending prayers to everyone who was/is at the #bostonmarathon.
Celebrities take to twitter to express shock
Dane Cook @DaneCook: My prayers with everyone in Boston right now after this atrocious act of violence!
Victoria Beckham @victoriabeckham: Just seen the news, my thoughts are with the people of Boston.
Justin Timberlake @jtimberlake: My thoughts and prayers are with everyone in the city of Boston and all of those affected by this terrible incident.
Mike Bloomberg @MikeBloomberg: Please keep the victims and their families in your thoughts and prayers.
Ricky Martin @ricky_martin: My heart goes out to all the victims of the explosion at the finish line.