The president is scheduled to speak at an interfaith service at 11 a.m., honoring the three people who were killed and the 170 who were injured in twin explosions near the finish line of the marathon.
Hundreds gathered at and around the Cathedral of the Holy Cross in Boston's South End long before the service, while uniformed police officers formed security lines around the church, said Reuters.
Also present will be Boston Mayor Thomas Menino and Massachusetts Gov. Deval Patrick. Cardinal Sean O'Malley will also speak at the service.
White House spokesman Jay Carney said on April 17 that Obama's message will be one of resolve.
"It will be one of the commonality that we all feel as Americans with the people of Boston and those who were visiting Boston for the marathon, and who both endured this horrific event and then demonstrated their bravery in its immediate aftermath," said Carney.
Meanwhile, a manhunt for a suspect spotted on video surveillance continues.