He called for "meaningful action" to prevent such shootings but did not say what it should be.
"The majority of those who died were children - beautiful, little kids between the ages of five and 10-years-old," Mr Obama said.
At that point he had to pause for several seconds to keep his composure and he wiped his eyes.
The scene in the White House briefing room was one of the most emotional moments of Mr Obama's presidency. Near him, two senior aides cried and held hands as they listened to the president.
Some 27 people, including 20 children, were killed when a gunman opened fire inside Sandy Hook Elementary School in Newtown. The shooter blasted his way through the building as young students cowered helplessly. The dead included the shooter.
Mr Obama began his comments with no greeting. He ended them with words of Scripture, walking away in silence. He recited the future milestones lost and had to pause again to gather his words. "They had their entire lives ahead of them - birthdays, graduations, weddings, kids of their own," the president said of those who were killed. He ordered that US flags be flown at half-staff on public grounds until Tuesday.
As the president received briefings about the shooting, his spokesman, Jay Carney, responded to questions about gun control and Mr Obama's campaign promises on the matter by saying "I don't think today is that day" for such a discussion.
Others, however, said it was. "If now is not the time to have a serious discussion about gun control and the epidemic of gun violence plaguing our society, I don't know when is," Representative Jerrold Nadler, a Democrat, said in a statement.
The president himself signalled a desire for action but he was not specific. "As a country, we have been through this too many times," Mr Obama said. "We're going to have to come together and take meaningful action to prevent more tragedies like this, regardless of the politics." During Mr Obama's time in office, mass shootings have shaken communities in Wisconsin, Texas and Colorado. The latest attack comes less than two weeks before the December 25 Christmas holiday. It appeared to be the nation's second-deadliest school shooting, exceeded only by the Virginia Tech University massacre in 2007.