The device was tossed over the fence at one point when about 1,000 to 1,500 Occupy DC protesters were demonstrating outside the White House, Secret Service spokesman George Ogilvie said.
He said authorities were taking appropriate measures to clear it. He also said that a majority of the protesters had left the area and that there had been no arrests.
President Barack Obama and wife Michelle had gone out to dinner and were not at the White House when the device was thrown over the fence but returned while the investigation was underway.
Members of the White House press pool were prevented from leaving the grounds.