Media playback is unsupported on your device Media caption Occupy DC protesters march towards the White House

The US Secret Service is investigating after a suspected smoke bomb was thrown over the fence of the White House during a mass Occupy protest.

No arrests were immediately reported after Tuesday night's incident and demonstrators dispersed from the area.

A Secret Service spokesman said the projectile was thrown as 1,000 to 1,500 Occupy DC protesters gathered outside.

President Barack Obama and the First Lady were reportedly out with friends for Michelle Obama's 48th birthday.

Journalists from the White House media pool were initially prevented from leaving the grounds during the incident.

The Secret Service spokesman, George Ogilvie, said measures were taken to remove the projectile.

Earlier in the day, four people were arrested as hundreds of Occupy protesters converged outside Capitol Hill to denounce what they see as the corrupting influence of money in politics.

Police denied the "Occupy Congress" activists entry to the congressional corridors during that rally, which took place as lawmakers returned from their winter break.