NEWTOWN, Conn. » President Barack Obama vowed Sunday to "use whatever power this office holds" to prevent massacres such as the slaughter at the school here that shocked the nation, hinting at a fresh effort to curb the spread of guns as he declared that there is no "excuse for inaction."

In a surprisingly assertive speech at a memorial service for the 27 victims, including 20 children, Obama said the country had failed to protect its young and that its leaders could no longer sit by idly because "the politics are too hard." While he did not elaborate on the action he will propose, he raised expectations that he will make a robust effort to stop gun violence.

The speech, a blend of grief and resolve, seemed to promise a significant change in direction for a president who has not made gun issues a top priority in four years in office. After each of three other mass killings during his tenure, Obama has renewed calls for legislation without exerting much political capital, but the definitive language Sunday may make it harder for him not to act this time.

"No single law, no set of laws can eliminate evil from the world," he said. "But that can’t be an excuse for inaction. Surely we can do better than this." He added that "in the coming weeks I’ll use whatever power this office holds" in an effort "aimed at preventing more tragedies like this."

"Because what choice do we have?" he added. "We can’t accept events like this as routine. Are we really prepared to say we’re powerless in the face of such carnage? 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?"

He added, "We can’t tolerate this anymore. These tragedies must end. And to end them we must change."

The service came as new details emerged about the terrifying moments at Sandy Hook Elementary School on Friday. Authorities said Sunday that the gunman, Adam Lanza, 20, shot his mother multiple times in the head before his rampage at the school and that he still had hundreds of rounds of ammunition left when he killed himself.

The president’s trip to Newtown came amid rising pressure to push for tighter regulation of guns in America, though aides tried to deflect that by saying it was a day for mourning. The president offered no specific proposals, and there were no urgent meetings at the White House over the weekend to draft legislation. But Obama said he would use the power of his office to confront the spate of shootings that have claimed so many lives, and that the administration does have a plan on the shelf, with measures drafted by the Justice Department. Among other things, Demo­crats said they would push to renew an assault rifle ban that expired in 2004 and try to ban high-capacity magazines like those used by the gunman in Newtown.

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 the ban on assault weapons and tighter background checks on gun purchasers.

Connecticut Gov. Dannel P. 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 U.S. 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.

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.

The Sunday public affairs television 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 Sunday.

"We had no takers," he said.

The NRA’s headquarters was closed Sunday, and a spokes­man could not be reached.

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.’"