Protestors linked to the "Occupy" movement apparently threw a smoke bomb over the White House fence, capping a day of protests in Washington, the Secret Service said.

No arrests were made after demonstrators from the Occupy DC group marched on the White House, and protesters were peacefully dispersed, as officers dealt with the device, a Secret Service spokesman said.

Witnesses said streets around the area were cordoned off as is normal on the frequent occasions when there is a minor security alert around the presidential mansion.

President Barack Obama and his wife Michelle were not in the White House compound at the time as they were at a nearby restaurant with friends celebrating the First Lady's 48th birthday.

Earlier, lawmakers returning from a three-week winter break were greeted by hundreds of protesters from the "Occupy" movement, who gathered outside the US Capitol building to decry the corrupting influence of money in politics.

Protesters from the self-proclaimed Occupy Congress group -- met by scores of uniformed police officers -- were denied entry to congressional corridors.

As they chanted amid a steady winter drizzle, the protesters wielded signs urging lawmakers to "Come out of your ivory tower and hear the people."

Another told lawmakers that "We are here to be heard."

By late afternoon, four people had been arrested. Three were charged with illegally crossing police lines and the fourth with assaulting a police officer, a Capitol Police spokesman told AFP.