An interfaith prayer service joined by President Barack Obama was held on Thursday following the attack on the Boston Marathon.

The mood was sombre as city leaders and residents came together at the Cathedral of the Holy Cross to pray and pay their respects.

Those in attendance included victims of the twin blasts, which killed three and injured over 170 others.

Mr Obama was defiant in his message, vowing that the perpetrators would be brought to justice and held accountable.

He told the congregation: "The spirit of this city is undaunted; the spirit of this country shall remain undimmed.

"Every one of us has been touched by this attack on your beloved city. Every one of us stands with you. Because after all it's our beloved city too.

"They sought to intimidate us, to terrorise us."

He added: "It should be pretty clear by now that they picked the wrong city to do it."

The Boston Marathon attacks killed Martin Richard, 8, Krystle Campbell, 29, and Lu Lingzi, a postgraduate student from China.

The cathedral was full for the service, with hundreds more gathered outside.

The FBI has released images of two male suspects seen carrying backpacks close to where the bombs exploded.

It has appealed to members of the public to help identify the men although they are being warned not to approach them.