The White House was locked down for more than an hour on Tuesday night as authorities investigated what appeared to be a smoke bomb tossed over the fence of the executive compound, a Secret Service spokesman said.
The device was thrown 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 gave the "all clear" later in the evening.
A majority of the protesters had left the area earlier and there had been no arrests, he said.
President Barack Obama and his wife, Michelle, went out to dinner to celebrate her birthday and were not at the White House when the incident began but returned while the investigation was under way.
Members of the White House press pool were prevented from leaving for about 45 minutes before they were escorted off the grounds. Pennsylvania Avenue in front of the White House was also closed during the investigation.
Demonstrators from the Occupy movement marched to the White House after earlier converging at the U.S. Capitol and congressional office buildings to protest against the influence of money on lawmakers.
Several hundred demonstrators, some from as far as Nevada and San Diego, staged rallies and attempts to meet lawmakers as they returned from a holiday break. One demonstrator was arrested for assaulting a police officer and three others for crossing a police line, a Capitol Police spokeswoman said.
The Occupy movement burst on the national scene in September in New York with its focus on income inequality in the United States.