White House locked down over bomb incident.

The White House in Washington was locked down for about an hour overnight as the US authorities investigated what appeared to be a smoke bomb that was thrown over the fence of the compound, RTE News reported.

The device is believed to have been thrown when over 1,000 protesters from the 'Occupy DC' movement were demonstrating outside the US President's residence.

A Secret Service spokesman said there had been no arrests.

President Barack Obama and wife Michelle were not at the location when the incident occured, but returned while the investigation was underway.

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 building was also closed during the investigation.