The US Supreme Court is considering arguments in a landmark case about the nation's gun laws, based on a challenge to a ban on handguns in Washington DC.
The nine justices will deliberate on whether the US Constitution protects an individual's right to have firearms, rather than a collective right.
The justices may also consider whether, if that is the case, DC's restrictions on gun ownership are "reasonable".
A ruling is expected in June, and may become an issue in November's election.
It is the first time in nearly 70 years that Americans' right to keep and bear arms has been considered by the Supreme Court.
As the session began, several dozen demonstrators supporting opposing sides of the gun law argument gathered outside the Supreme Court building.
Washington DC has some of the strictest gun control laws in the United States.
Since 1976, there has been a ban on the private possession of handguns in the capital, as well as a requirement to have rifles or shotguns locked or dismantled.
The challenge to those rules has been brought by a federal building security guard, Dick Heller.
He argues that if he is allowed a handgun at work, he has a constitutional right to have one at home for self-defence.
DC city council argues than the ban is justified because "handguns have no legitimate use in the purely urban environment of the District of Columbia".
The debate, which has raged for many years, is centred on whether the Second Amendment of the US Constitution protects an individual's right to possess guns, or simply a collective right for an armed militia.
While a decision is months away, several conservative members of the court seemed ready to back the idea that the amendment preserves an individual right.
Justice Anthony Kennedy left little doubt about his view when he said that the Second Amendment gives "a general right to bear arms".
And Chief Justice John Roberts, questioning the capital's handgun laws if individuals were ruled to have a constitutional right to bear arms, asked: "What is reasonable about a total ban on possession?"
However, Stephen Breyer, seen as one of the more liberal members of the court, brought up the city's argument that the ban was needed for public safety.
He cited figures suggesting 200 to 300 people are killed and 1,500 to 2,000 wounded annually in DC as a result of gun violence.
"In light of that, why isn't a ban on handguns... a reasonable or proportionate response on behalf of the District of Columbia?" he asked.
The case before the Supreme Court is being closely watched and has attracted dozens of briefs from outside groups arguing their point of view.
"This may be one of the only cases in our lifetime when the Supreme Court is going to interpret an important provision of our Constitution unencumbered by precedent," Georgetown University law professor Randy Barnett told the Associated Press news agency.
The country - including the Bush administration - is split on the issue.
US Solicitor General Paul Clement has argued that while individuals may have the right to own a gun, they are still subject to reasonable government intervention.
Others, including Vice President Dick Cheney and John McCain, the presumptive Republican presidential nominee, are urging the court to take a stronger stand in favour of gun rights, and overturn the Washington ban.
Organisations also backing gun rights include groups as diverse as Pink Pistols and Gays and Lesbians for Individual Liberty, Jews for the Preservation of Firearms Ownership, as well as the powerful gun lobby, the National Rifle Association (NRA).
"More anti-gay hate crimes occur in the home than in any other location," the Pink Pistols argued in their brief to the court, arguing that guns should be allowed in the home for self-defence.
The groups on the other side of the argument include law enforcement agencies, the American Bar Association, and coalitions against domestic violence.
They fear that easing access to handguns will lead to a rise in murder rates.
BBC Washington correspondent Jamie Coomarasamy says that the Supreme Court's eventual ruling could have reverberations across the US.