Washington (CNN) -- A "smoking object" was thrown over the White House fence late Tuesday, and authorities have closed the north gate of the residence while the object is investigated, U.S. Secret Service spokesman George Ogilvie said.
The president and his wife were not at home; they were celebrating the first lady's birthday at a restaurant a few blocks away.
The unidentified object was thrown during a protest, Ogilvie said.
Occupy protesters in Washington rallied at the White House, on Capitol Hill and at the Supreme Court on Tuesday, railing against corporate control at the nation's most powerful political institutions.
At least one protester was arrested for assault of a police officer, according to U.S. Capitol Police.
Occupy spokesman Ben Zucker said the man was arrested after failing to comply with an order to move into a sanctioned space.
At the demonstration in front of the White House, protesters tied "wish notes" in the shape of hearts along a fence.
The Occupy movement began in September in Lower Manhattan, before spreading to communities around the country and the world as a call to action against unequal distribution of wealth and other issues.
CNN's Greg Seaby, Skip Nocciolo and Stacey Samuel contributed to this report.