In a week marked by unspeakable tragedy, President Obama traveled to Boston today to participate in an interfaith service honoring the marathon bombing victims and the first responders who rushed to their aid. First Lady Michelle Obama, Sen. William "Mo" Cowan and other members of the Massachusetts congressional delegation accompanied him.

"It was a beautiful day to be in Boston, a day that explains why a poet once wrote that this town is not just a capital, not just a place. Boston, he said, is the perfect state of grace," Obama said. "And then, in an instant, the day's beauty was shattered. A celebration became a tragedy. And so we come together to pray and mourn and measure our loss. But we also come together today to reclaim that state of grace, to reaffirm that the spirit of this city is undaunted and the spirit of the country shall remained undimmed."

The president, who shared fond memories of his time there as a young law student, praised the city's residents kindness, reminded them of their resiliency and offered reassurance that the rest of the nation is there to support them.

"We will all be with you as you learn to stand and walk and, yes, run again. Of that I have no doubt," he said. "You will run again. You will run again because that's what the people of Boston are made of."

Obama also issued a warning to the those responsible for the bombings.

"Yes, we will find you. And yes, you will face justice. We will find you. We will hold you accountable," he said. "But more than that, our fidelity to our way of life, for a free and open society, will only grow stronger, for God has not given us a spirit of fear and timidity, but one of power and love and self-discipline."

The prayer service was held in the Cathedral of the Holy Cross in the city's South End neighborhood, less than a mile from the marathon finish line. Former governors Mitt Romney, William Weld and Michael Dukakis were among the dignitaries in the audience. Boston Globe reported that people began lining up at 6:30 a.m., hoping to get one of the 2,000 seats. Many of those who couldn't get in remained outside the cathedral to share in the moment.

Rev. Liz Walker of Roxbury Presbyterian Church, a former local news anchor, opened the service, which featured scripture readings, a performance by the Boston Children’s Chorus and reflections and prayers from clergy of various faiths. The internationally renowned cellist Yo-Yo Ma performed a musical selection.

The president was preceded by Massachusetts Gov. Deval Patrick, who expressed gratitude to the first responders and ordinary citizens who "in the aftermath of such senseless violence let their first instinct be kindness."

"Massachusetts invented America," he said to applause, adding that the nation has over time and with struggle defined the ideals of equality, opportunity, freedom and fair play.

"An attack on our civic ritual like the marathon, especially on Patriots Day, is an attack on those values. And just as we cannot permit darkness and hate to triumph over our spiritual faith, so we must not permit darkness and hate to triumph over our civic faith," Patrick said. "That cannot happen. And it will not. So we will recover and repair. We will grieve our losses and heal. We will rise and we will endure. We will have accountability without vengeance; vigilance without fear and we will remember I hope and pray long after the buzz of Boylston Street is back and the media has turned its attention elsewhere that the grace this tragedy exposed is the best of who we are."

The president is expected to meet with the families of the victims injured or killed in Monday's blast.

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(Photo: Spencer Platt/Getty Images)