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.