President Barack Obama and Vice President Joe Biden have introduced new gun-control proposals, saying the nation has an obligation to do everything possible to prevent mass shootings.
In announcing the gun-control proposals Vice President Joe Biden invoked the memory of the 20 children and six adults killed last month at Sandy Hook Elementary School in Connecticut.
"No one will know for certain if this senseless act could have been prevented, but we all know we have a moral obligation, a moral obligation, to diminish the prospect that something like this could ever happen again," Biden said.
Obama and Biden, who headed the gun-violence task force that researched the proposals, spoke to a packed auditorium in the Old Executive Office Building next to the White House.
In the audience were family members of victims of the Connecticut shooting. Behind Obama and Biden were four children who wrote letters to the president after the tragedy.
Obama said protecting children should not be a divisive challenge.
"In the month since 20 precious children and six brave adults were violently taken from us at Sandy Hook Elementary, more than 900 of our fellow Americans have reportedly died at the end of a gun - 900 in the past month. And every day we wait that number will keep growing," he said.
Obama wants Congress to approve universal background checks for anyone trying to buy a gun, limits on high-capacity ammunition magazines, and to ban assault-style weapons.
He signed 23 executive orders aimed at strengthening the background check system, developing emergency response plans for schools and religious institutions, and addressing mental health issues linked to gun violence.
The president said he respects the Second Amendment right to own guns and appealed to Americans, including responsible gun owners, to support effective action.
"There are millions of responsible law-abiding gun owners in America who cherish their right to bear arms for hunting or sport or collection," Obama said. "I also believe that most gun owners agree that we can respect the Second Amendment while keeping an irresponsible law-breaking few from inflicting harm on a massive scale."
Related video report by Kent Klein
The National Rifle Association (NRA) has accused Obama of attacking the Second Amendment and vows to fight gun control legislation on Capitol Hill.
Monday the group issued a video criticizing the president's opposition to putting more armed guards in schools, and noting that the president's daughters receive Secret Service protection.
The NRA video asked, "Are the president's kids more important than yours? Then why is he skeptical about plutting armed security in our schools when his kids are protected by armed guards at their school?"
White House spokesman Jay Carney condemned the ad, saying making "the safety of the president's children the subject of an attack ad is repugnant and cowardly."
City and police officials from Philadelphia and New York, representatives of gun control groups, and family members of gun violence victims spoke with reporters.
Annette Nance-Holt lost her son to gun violence in Chicago, President Obama's home town.
"We need the American public to speak up about this issue and speak to the legislators to change their minds and change their hearts to do the right thing, because you too can be me and you can lose your only child," she said.
"In order for change to happen on this issue the American public is going to need to make its voice heard," said Dan Gross, who heads the Brady Center to Prevent Gun Violence.
Obama said he knows the gun violence battle will be difficult, predicting that opponents will warn of a "tyrannical all-out assault on liberty" and try to block common sense reform.
He said change will come only if Americans, including those in areas where there is a strong tradition of gun ownership, push representatives in Congress and say "this time must be different."