President Barack Obama assured the grieving, shell-shocked Newtown community on Sunday that "you are not alone" and vowed sternly to wield "whatever power this office holds" in a quest to prevent future mass shootings.

"We can't tolerate this anymore," Obama said from behind a podium on the stage of a Newton High School auditorium, as adults wept, or hugged, or sat quietly, many hugging small children. "These tragedies must end. And to end them, we must change."

"In the coming weeks, I'll use whatever power this office holds to engage my fellow citizens -- from law enforcement, to mental health professionals, to parents, and educators -- in an effort aimed at preventing more tragedies like this, because what choice do we have?"he said.

The speech, broadcast nationwide, offered the bold suggestion that Obama might engage lawmakers on the subject of gun control -- a topic that has not been among his top priorities during his presidency.

"We can't accept events like this as routine. Are we really prepared to say that we're powerless in the face of such carnage?" Obama said. That the politics are too hard? Are we prepared to say that such violence visited on our children year, after year, after year is somehow the price of our freedom?"

There were sobs from the crowd as the president read the first names of the 20 children slaughtered at Sandy Hook Elementary School on Friday and paid tribute to the six adults who died defending them. Twenty-six candles in twenty-six shining glass vases shone from the base of the podium.

Obama anticipated — and dismissed — some of the time-honored arguments against stricter restrictions on guns. "We will be told that the causes of such violence are complex, and that is true," he said. "No single law no set of laws can eliminate evil from the world or prevent every senseless act of violence in our society."

"But that can't be an excuse for inaction. Surely we can do better than this," he said.

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Across the country, people grieved for the 20 children — six and seven years old — and six adults killed in one of the worst mass shootings in America's history.

In Newtown and elsewhere, mourners gently piled notes, stuffed animals and American flags, balloons and flowers, in makeshift memorials where candles fluttered.

New York Giants wide receiver Victor Cruz played wearing a shoe that read "R.I.P. Jack Pinto" in black marker, an homage to a child slain in the massacre. Flags from coast to coast flew at half-staff. As the president's motorcade climbed the hill up the school, he could glimpse a few homes with Christmas lights -- but most were dark.

"Here in Newtown, I come to offer the love and prayers of a nation," the president said. "I am very mindful that mere words cannot match the depths of your sorrow, nor can they heal your wounded hearts."

"I can only hope it helps for you to know you are not alone in your grief that our world too has been torn apart. That all across this land of ours, we have wept with you. We've pulled our children tight," Obama said. "And you must know that whatever measure of comfort we can provide, we will provide. Whatever portion of sadness that we can share with you to ease this heavy load, we will gladly bear it."

In the auditorium where the president spoke, the audience included a large number of elementary school-age children, some carrying cuddly toys like teddy bears, according to pool reporter Stephen Collinson of Agence France-Presse.

Before the service, Obama met privately for more than an hour with families of the victims and emergency workers who responded to the crisis. As those workers entered the auditorium, the crowd erupted in a standing ovation. Some traded long hugs with members of the audience.

"We needed this. We needed to be together," said Rev. Matt Crebbin, the senior minister at Newtown Congregational Church. "These darkest days of our community shall not be the final word heard from us."

Connecticut Governor Dannel Malloy, describing his meeting with Obama, said that the president had called Friday "the most difficult day of his presidency."

By 4 p.m., the line cars trying to reach the interfaith vigil stretched more than 2 miles from Newtown High School back through Sandy Hook -- and its growing makeshift memorial -- to Saint Rose church, the site of several vigils for (and hoax threats related to) Friday's massacre.

In Sandy Hook center, a lawn displayed lights with the phrase "FAITH. HOPE. LOVE." Across the street, a sign wrapped around a street lamp read, "Heaven must have been short on 27 angels."

The president spoke about the shooting on Friday, his voice choked with emotion, one finger wiping away tears as they welled up. He vowed to "take meaningful action, regardless of the politics" to try to prevent future such tragedies. But hours before, White House press secretary Jay Carney had decreed that "today's not the day" to discuss possible gun control measures.

The Obama administration has reportedly considered new gun restrictions in the past, only to shelve them.

The White House has shied from seeking tough new action from Congress — where new restrictions on gun purchases would likely run into stiff Republican opposition.

Obama's speech was the fourth in his presidency to memorialize a mass shooting. After the January 2011 rampage in Tucson, AZ, where then-Rep. Gabrielle Giffords was critically injured, the president spoke at a memorial for the six people killed, including Christina Taylor Green, 9.

Dylan Stableford contributed from Newtown