BOSTON - President Barack Obama declared "there is a piece of Boston in me" as he paid tribute to a city shaken by what he has called an act of terror.

The president addressed an interfaith service in the aftermath of Monday's twin blasts that killed 8-year-old Martin Richard, 29-year-old Krystle Campbell and 23-year-old Lingzi Lu, and injured more than 170 others at the Boston Marathon finish line.

Obama said a day of beauty was shattered when a celebration became a tragedy, but triumphantly declared, "You will run again!"

Boston Mayor Thomas Menino, Massachusetts Gov. Duval Patrick and members of the clergy also spoke at the Cathedral of the Holy Cross in Boston's South End.

Security was heavy surrounding the city's main Roman Catholic church. Streets were blocked off as people lined up before dawn hoping to get one of the roughly 2,000 seats inside, but by 9 a.m., they were being turned away.

AP wire services contributed to this report