Barack Obama has called for tighter background checks on US gun-owners, making his first foray into the politically charged gun control debate since the Colorado shooting.
He also questioned whether the widely cited second amendment, the "right to bear arms", should include assault rifles, such as the semi-automatic AR-15 with a 100-round drum magazine that was used by the Denver gunman.
Referring to a similar weapon, Obama told a civil-rights group, "I believe that a lot of gun owners would agree that AK-47s belong in the hands of soldiers, not in the hands of criminals; that they belong on the battlefield of war, not on the streets of our cities."
The remarks come less than a week after the shootings at a midnight screening of the new Batman film.
They follow calls from prominent gun control advocates including Michael Bloomberg, the mayor of New York, for the two presidential candidates to hold an open and honest debate over guns.
Both Obama and Mitt Romney, his Republican opponent, have supported curbs on assault rifles in the past. But they have been wary of public opinion polls that have shown waning support for tougher gun laws in recent years.
"Every day and a half the number of young people we lose to violence is about the same as the number of people we lost in that movie theatre," Obama added in remarks to the National Urban League in New Orleans.
But while the president pledged to "arrive at a consensus around violence reduction" analysts said there was little prospect of significant change in US gun laws.
Democrats remain scarred by the memory of heavy midterm election defeats in 1994 after president Bill Clinton forced through a 10-year ban on assault weapons.
It provoked a furious backlash from gun lobby groups.
Romney, whose party's grass roots are staunch defenders of the right to bear arms, said on his visit to London that tougher laws were not the way to address America's epidemic of gun crime.
"I don't happen to believe that America needs new gun laws," he told NBC news. "A lot of what this … young man did was clearly against the law.
"But the fact that it was against the law did not prevent it from happening."
Romney added: "We can sometimes hope that just changing the law will make all bad things go away. It won't. Changing the heart of the American people may well be what's essential."
The National Rifle Association, the powerful pro-gun lobby group, has so far refused to join the debate on gun controls in the wake of the massacre.
"The NRA believes that now is the time for families to grieve and for the community to heal," said Andrew Arulanandam, the group's director of public affairs.
In Denver, as the first funerals and memorial services for the 12 people killed took place, reports emerged that several of those wounded in the attack fear they will be unable to meet their medical bills. The family of a 23-year-old aspiring comic, Caleb Medley, who is in a critical condition with a head wound and whose wife Katie gave birth to their first child on Tuesday, announced that they were seeking online donations.
Family and friends said they had set a goal of raising $500,000 [pounds 320,000] to cover his hospital bills and other expenses and were more than halfway there.
