© AFP Photo/Brendan SmialowskiPeople listen to a program outside the Smithsonian's Air and Space Museum during a demonstration on the National Mall in Washington, DC, June 2011.
The incident occurred at The Smithsonian's National Air and Space Museum around 3:15 pm (1915 GMT), after hundreds of activists had marched from Freedom Plaza, near the White House, along the National Mall towards the US Capitol.
Some of those in the demonstration were affiliated with the Occupy DC protest group that sprung up earlier this week as a spin-off of the larger Occupy Wall Street movement in New York, angered at "corporate greed."
Smithsonian spokeswoman Linda St. Thomas said a group of the protesters were confronted when they tried to push through the doors and enter the Air and Space Museum.
"When they were told they couldn't bring the banners they were carrying inside one security officer used pepper spray," she said.
"There were a couple of hundred protesters in the area at the time and Washington police were called. There was one arrest that I know of," she added, noting that the museum closed its doors two hours early because of the clash.
Occupy DC, a young group of several dozen protesters, is camping out at McPherson Square on Washington's K Street, where many political lobbyists have their offices.
On Friday its members greeted commuters with placards denouncing corporate greed. Later they mingled in small groups and swapped experiences with Occupy Wall Street participants who have come down to the nation's capital.
Protesters in New York, angered by government bailouts of banks based on Wall Street, began to gather on September 17, leading to continuous, increasingly high-profile demonstrations.
The protests have since spread to other US cities.
After all, it's one of the "Three Cities."
The three stars of the Washington DC flag are sometimes said to represent the 3 major city-states of the Western world: The Vatican, the City of London, and the District of Columbia. However, its designers claim it comes directly from the Washington family coat of arms, extant images of which exist at Selby Abbey in England, said to date from the 1300's.