"We can't tolerate this anymore," he said. "These tragedies must end. And to end them, we must change."
Obama said that he will use the power of his office to confront the spate of shootings that have claimed so many lives, many of them children. He was not specific, but he made it clear that he will pursue change in the face of political opposition that has stopped new gun laws for years.
"I'll use whatever power this office holds to engage my fellow citizens," he said, "in an effort aimed at preventing more tragedies like this. Because what choice do we have?"
Obama said the nation is failing at what he called "our first task," which he said was to care for the children of the nation.
"It's our first job. If we don't get that right, we don't get anything right," he said, asking: "Can we truly say, as a nation, that we are meeting our obligations? Can we honestly say that we are doing enough to keep our children - all of them - safe from harm?"
Obama asked whether the nation is doing enough to give all children a chance at a good life with "happiness and with purpose."
"If we are honest with ourselves, the answer is no," he said. "We are not doing enough, and we will have to change."
In a high school auditorium that might one day have showcased the musical performances of the children cut down at the Sandy Hook Elementary School nearby, the president listened as clergy offered prayers for the 27 people who were killed, 20 of them young children.
For Obama, the last three days have been an extraordinary moment of his presidency - one marked by the raw grief of his reaction at the White House on Friday and the tantalizing but vague suggestion that he might confront the scourge of gun violence.
The service came as new details emerged about the terrifying moments at the elementary school on Friday that shocked the nation. Authorities said Sunday that the gunman, Adam Lanza, shot his mother multiple times in the head before his rampage at the school and that he still had hundreds of rounds left when he killed himself. Connecticut Gov. Dannel P. Malloy said Lanza shot himself as the police were closing in, suggesting that he may have intended to take more lives had he not been interrupted.
The president's trip here came amid rising pressure to push for tighter regulation of guns in America. While aides tried to deflect that by saying it was a day for mourning, the streets outside the memorial service and the airwaves across the nation were filled with voices calling for legislative action. By contrast, the National Rifle Association and its most prominent supporters in Congress were largely absent from the public debate.
"These events are happening more frequently," Sen. Joseph I. Lieberman, I-Conn., said here before the service began, "and I worry that if we don't take a thoughtful look at them, we're going to lose the pain, the hurt and the anger that we have now."
Lieberman, who is retiring, called for a national commission on mass violence, the reinstatement of a ban on assault weapons that expired in 2004 and tighter background checks on gun purchasers.
"If you go to a gun show, or buy a gun from some antique dealer, you're not checked at all," he said.
The state's other leaders added their endorsements of more laws.
"I'm hearing from the community, as well as my colleagues in law enforcement, we need to do something," Sen. Richard Blumenthal, a Democrat, said on "This Week" on ABC. Malloy said on CBS' "Face the Nation" that when someone can burst into a building with "clips of up to 30 rounds on a weapon that can almost instantaneously fire those, you have to start to question whether assault weapons should be allowed to be distributed the way they are in the United States."
The grieving in this small New England town, aired nonstop on national television, added emotional energy to an escalating debate about the role of firearms in the United States. The calls for more gun control that typically follow such events have evolved this time around into particular pressure on a newly re-elected Democratic president who has largely avoided the issue during four years in the White House.
Obama has long supported the restoration of the assault weapon ban, which first passed in 1994 only to set off a backlash among supporters of gun rights and help cost Democrats control of Congress.
Mayor Michael R. Bloomberg of New York, appearing on the NBC program "Meet the Press," all but demanded that Obama confront the prevalence of firearms in the nation. Bloomberg, an independent who gave his support to the president shortly before the November election partly on the basis of gun control, bluntly said he expected more of Obama.
"It's time for the president to stand up and lead," he said. "This should be his No.1 agenda. He's president of the United States. And if he does nothing during his second term, something like 48,000 Americans will be killed with illegal guns" in the next year.
Bloomberg added that it was no longer enough that Obama shared his position on banning assault weapons.
"The president has to translate those views into action," he said. "His job is not just to be well-meaning. His job is to perform and to protect the American public."
The Sunday shows were filled with politicians, mainly Democrats like Sen. Dianne Feinstein of California, demanding stronger gun control while supporters of gun rights were noticeably absent. David Gregory, the moderator of "Meet the Press," said his show invited 31 senators who support gun rights to appear on Sunday.
"We had no takers," he said.
The NRA's headquarters was closed Sunday and a spokesman could not be reached. A spokesman for Sen. Mitch McConnell of Kentucky, the Republican minority leader, said he had no comment, while Rep. Eric Cantor of Virginia, the Republican House majority leader, could not be reached.
Robert A. Levy, chairman of the libertarian-leaning Cato Institute and one of the organizers behind a Supreme Court case that enshrined a Second Amendment individual right to own guns, said Sunday that with more than 250 million guns already in circulation in the United States, restrictions on new guns would make little difference. He said by email that tough gun laws did not stop a mass shooting in Norway or regular violence in places like the District of Columbia.
"I'm skeptical about the efficacy of gun regulations imposed across the board - almost exclusively on persons who are not part of the problem," he said. "To reduce the risk of multivictim violence, we would be better advised to focus on early detection and treatment of mental illness. An early detection regime might indeed be the basis for selective gun access restrictions that even the NRA would support."
In an interview on Sunday, Rep. Peter T. King, R-N.Y., chairman of the House homeland security committee, renewed his call for tougher restrictions, including banning assault weapons, requiring background checks for firearm purchases at gun shows and prohibiting gun sales to anyone on the nation's terrorism watch list.
But King said he doubted that the shootings in Connecticut would alter the gun debate in Congress, saying that outside the Northeast a gun culture exists that is resistant to any kind of firearms regulation.
"I hope I am wrong," he said, "but I don't think it will have a major impact on the debate in Congress. We've had a number of gun tragedies in recent years without any action being taken."
Attention focused mainly on Obama, who has shied away from a major push on gun control even after events like the shooting of Rep. Gabrielle Giffords in Tucson, Ariz., last year and the mass killing at a movie theater in Aurora, Colo., this year. Some Democrats said the number of children involved in the Newtown massacre might change the dynamics but only if the president seizes the moment.
"Nothing's going to happen here unless Obama decides to put it front and center," said Steve Elmendorf, who was a top Democratic congressional aide in 1994 when lawmakers passed the last major gun control measure. "He's not running for re-election. This is one of those moments where you have to decide, 'I'm not going to sit here and examine the politics and I'm going to do what's right."'
"But nobody should be under any illusion that it's easy," Elmendorf added. Recalling Giffords shooting, he noted, "One of their own got shot and nobody did anything about it."