BOSTON, Massachusetts (AFP) - US President Barack Obama vowed on Thursday that investigators would find the bombers who attacked the Boston marathon and make them "accountable".

"Yes, we will find you, and yes, you will face justice," Mr Obama told a special inter-faith service in the city, three days after the attacks which killed three people and injured about 180.

"We will find you, we will hold you accountable," he added in his keynote speech on a special visit to show national solidarity with what he called "one of the world's great cities.

"If they sought to intimidate us, to terrorise us," Mr Obama said, then "it should be pretty clear by now that they picked the wrong city to do it."

Mr Obama's message drew several ovations by the 2,000 congregation in Boston's Cathedral of the Holy Cross.