NEWTOWN, Conn. - A solemn, stern President Obama told the shattered Newtown community Sunday night they were not alone and vowed to push a national discussion on how to avoid mass killings involving firearms.

For the fourth time in as many years, Obama addressed an audience mourning such a tragedy.

Fighting to control his emotions, the president read the names of the 20 murdered children and memorialized the six adults slain Friday at Sandy Hook Elementary School. He promised their deaths would result in action.

"We can't tolerate this anymore, these tragedies must end," Obama said. "And to end them, we must change."

Families from throughout Connecticut streamed their way into this small New England town for the vigil at Newtown High School -- a school once attended by Adam Lanza, who committed the massacre.

Some trekked for more than a mile in a cold, misty rain and waited in long lines to share in the comfort of their community and hear from Obama and church leaders. When too many showed up, a spillover room was added.

Children clutched teddy bears as parents cried. First responders were given a standing ovation from the auditorium. Moments later, the crowd was silent as a piano played "Amazing Grace."

One by one, local priests, rabbis and ministers led the audience in prayer. In front of them was a table with a lit candle for each victim. Before taking the stage, Obama met privately with families of the victims and first responders.

For Obama, playing the role of comforter in chief has become all too common. The nation looked to him for healing in the wake of mass shootings at Fort Hood, Texas, Tucson, Ariz., where Rep. Gabrielle Giffords was gravely injured, and most recently, a movie theater in Aurora, Colo.

But with 20 children slain just days before Christmas, this represented a completely new test for the president.

"I haven't seen anything like this since he took office," said Charles Walcott, a political scientist at Virginia Tech who focuses on the presidency. "But he's handled it quite well. He's approaching the situation as a father, and you can tell the emotion is genuine -- that's what the American people are looking for."

Just as he did after the shooting in Tucson, Obama wrote most of his remarks, White House officials said. The president's promise to take action was widely seen as a call for reforming the nation's gun laws, though he did not include any specifics in his Sunday remarks.

In the wake of the tragedy, hundreds of thousands of Americans have signed petitions calling for stricter gun laws. A petition posted on the White House website had nearly 130,000 signatures as the president addressed the Connecticut crowd Sunday evening.

In Newtown, the shaken town was looking to heal.

"You need your president at a time like this," said Louise Bouchard, of nearby Brookfield, outside the high school.

Obama sought to ensure them that America would share their suffering and help them heal.

"As a community, you've inspired us, Newtown. In the face of indescribable violence, in the face of unconscionable evil, you've looked out for each other, and you've cared for each other, and you've loved one another," Obama said. "This is how Newtown will be remembered."