The White House was locked down for more than an hour after a smoke bomb was reportedly tossed over the fence of the White House compound.
The bomb was thrown during an Occupy DC protest outside the White House, according to the US Secret Service.
The authorities are investigating. No one has been detained, according to reports.
A Secret Service spokesperson said all protesters had vacated the area when the suspected smoke bomb was thrown.
President Barack Obama and wife Michelle were not at the White House at the time of the incident but returned later.
The first couple were out to celebrate Michelle's 48th birthday a few blocks away from the White House.
The Occupy protests, which began in September in Lower Manhattan, have spread across the country and around the world. Demonstrators have been protesting against unequal distribution of wealth.
The White House is under the protection of the US Secret Service and the United States Park Police. The air space around the White House is protected by surface-to-air missiles.
Breaches of security are rare. In 1994, a small plane crashed inside the compund.