Christians are praying after two explosions at the Boston Marathon yesterday left three people dead and at least 140 injured.
US media reports that one of the victims was an eight-year-old boy. There are reports of horrific injuries, with some people losing limbs.
President Barack Obama made a televised address in which he vowed to bring the perpetrators to justice.
"We will find out who did this. We'll find out why they did this," he said.
"Any responsible individuals, any responsible groups, will feel the full weight of justice."
At present, it is unclear who carried out the attack but a White House official told Fox News: "When multiple devices go off, that's an act of terrorism."
Christians have taken to Twitter to express their sympathies and ask for prayers with the hashtags #prayforboston and #BostonMarathon
They include theologian Vicky Beeching, who described events as "tragic".
She tweeted: "Thinking of & praying for all :("
Others tweeting their prayers include:
Joyce Meyer @JoyceMeyer
Please join with us in praying for those injured by the explosions at the #BostonMarathon
The Salvation Army @salvationarmyuk
Our thoughts and prayers are with all those affected by the #bostonmarathon tragedy today
Sr Catherine Wybourne @Digitalnun
Prayers for those killed/injured in the #BostonMarathon bomb attack
Fr James Bradley @frjamesbradley
My aunt and family are safe. Thanks be to God and thanks for your prayers. Please pray for those not so lucky. #BostonMarathon
World Vision USA @WorldVisionUSA 4h
Join us and @TeamWorldVision as we pray for fellow runners and their families affected by explosions at the #BostonMarathon
Alastair McCollum @revdal 12h
Prayers for all those caught up in #BostonMarathon disaster. Heartbreaking that anyone would do this to fellow Human Beings. Kyrie Eleison
The Archbishop of Boston, Cardinal Sean O'Malley expressed his "deep sorrow" over the "senseless acts of violence" perpetrated at the marathon.
"Our prayers and concern are with so many who experienced the trauma of these acts, most especially the loved ones of those who lives were lost and those who were injured, and the injured themselves," he said.
"In the midst of the darkness of this tragedy we turn to the light of Jesus Christ, the light that was evident in the lives of people who immediately turned to help those in need today.
"We stand in solidarity with our ecumenical and interfaith colleagues in the commitment to witness the greater power of good in our society and to work together for healing."
The Episcopal Diocese of Massachusetts, which covers Boston, is holding a special prayer service at the city's Cathedral Church of St Paul today.
The service will be led by Bishop Gayle E Harris.
Local Christians were among the marathon runners. Trinity Church in Copley Square reports that the marathon runners on its charity team are safe.
Church personnel based in the city centre have reported chaos around the site of the explosions and limited mobility.
The Cathedral Church of St Paul was forced to cancel its regular Monday evening community meal because volunteer servers were unable to make it into the city.
Episcopal Bishop Tom Shaw has requested prayers for the city and all those affecting, as well as emergency responders.
Presiding Bishop of the US Episcopal Church Katharine Jefferts Schori has offered the following prayer:
Gracious God, you walk with us through the valley of the shadow of death. We pray that the suffering and terrorized be surrounded by the incarnate presence of the crucified and risen one. May every human being be reminded of the precious gift of life you entered to share with us. May our hearts be pierced with compassion for those who suffer, and for those who have inflicted this violence, for your love is the only healing balm we know. May the dead be received into your enfolding arms, and may your friends show the grieving they are not alone as they walk this vale of tears. All this we pray in the name of the one who walked the road to Calvary.
In a pastoral response to the explosions, Mars Hill Church Pastor David Fairchild said the root cause was an "insane rebellion against our Creator God".
"Sadly, in a split second, people and families have been shattered and forever changed by this terrible tragedy," he said.
"The most powerful weapon at our disposal against evil is the hope-filled prayer of God's people. Join me in praying for those in attendance as well as the families of those affected by this heinous act.
"Also, please pray for the policemen, firemen, and medical personnel treating and protecting the victims of this evil act—may God give these men and women helpful words for hurting people."