Dozens of protesters mingled with tourists and waved signs saying "Ban the Washington elitists, not our guns" or "The NRA helps criminals and terrorist buy guns."
Members of the Brady Campaign to Prevent Gun Violence chanted "guns kill" as followers of the Second Amendment Sisters and Maryland Shall Issue.Org shouted "more guns, less crime."
A line to get into the court for the historic arguments began forming two days earlier and extended more than a block by early Tuesday.
The high court's first extensive examination of the Second Amendment since 1939 grew out of challenge to the District of Columbia's ban on ownership of handguns.
Anise Jenkins, president of a coalition called Stand Up for Democracy in D.C., defended the district's 32-year-old ban on handgun ownership.
"We feel our local council knows what we need for a good standard of life and to keep us safe," Jenkins said.
Genie Jennings, a resident of South Perwick, Maine, and national spokewoman for Second Amendment Sisters, said the law banning handguns in Washington "is denying individuals the right to defend themselves."
The court has not conclusively interpreted the Second Amendment in the 216 years since its ratification. The basic issue for the justices is whether the amendment protects an individual's right to own guns or whether that right is somehow tied to service in a state militia.
Even if the court determines there is an individual right, the justices still will have to decide whether the District's ban can stand and how to evaluate other gun control laws. This issue has caused division within the Bush administration, with Vice President Dick Cheney taking a harder line than the administration's official position at the court.
The local Washington government argues that its law should be allowed to remain in force whether or not the amendment applies to individuals, although it reads the amendment as intended to allow states to have armed forces.
The City Council that adopted the ban said it was justified because "handguns have no legitimate use in the purely urban environment of the District of Columbia."
Dick Anthony Heller, 65, an armed security guard, sued the District after it rejected his application to keep a handgun at his home for protection. His lawyers say the amendment plainly protects an individual's right.
The 27 words and three enigmatic commas of the Second Amendment have been analyzed again and again by legal scholars, but hardly at all by the Supreme Court.
The amendment reads: "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."
The last Supreme Court ruling on the topic came in 1939 in U.S. v. Miller, which involved a sawed-off shotgun. Constitutional scholars disagree over what that case means but agree it did not squarely answer the question of individual versus collective rights.
Chief Justice John Roberts said at his confirmation hearing that the correct reading of the Second Amendment was "still very much an open issue."