While the Obamas were dining at one of Washington's finest steak houses,  Occupy DC protesters gathered in front of the White House and for a couple of hours, drew dozens of police cars to Pennsylvania Avenue and briefly kept the press on lockdown inside the building.

The cause of the commotion is unclear but may have been a smoke bomb or firecracker  hurled by a protester over the White House fence from Pennsylvania Avenue.

When the press covering the president's dinner trip this evening returned to the White House they were told they could not leave the building. Secret Secret told the reporters they were keeping them there while they investigated a "smoke bomb" that protesters may have thrown over the fence and onto the White House grounds.

The Associated Press' Mark Smith reports: "Upon returning to the WH the (press pool) learned of the Secret Service report that a smoke bomb had earlier been tossed over the fence during a protest by Occupy protesters. There were no arrests or injuries. However, because of the incident pool members were not being allowed to leave the grounds."

Shortly after 8 p.m., OccupyDCKst tweeted, "Someone just chucked something over the fence, it's on fire. People are saying its a firecracker."

The Secret Service told the ABC affiliate in Washington, WJLA-TV, that it was indeed a smoke bomb.  The Obamas weren't at the White House at the time of the incident.

For some time this evening, the Secret Service  had Pennsylvania Avenue closed, beyond the normal restrictions in that part of Pennsylvania Avenue, which is limited to pedestrians only.

The Obamas' evening out was in celebration of  first lady Michelle Obama's 48th birthday. She was celebrating tonight with her husband and close friends at BLT Steak in Washington, just blocks from the White House.

Tonight's dinner continues a birthday tradition. In years past, the first lady has celebrated at some of DC's finest restaurants, including Wolfgang Puck's The Source and Restaurant Nora.

The president, who is two years older than his wife, joked about her age today.

"When we first married, it was a little controversial that she was 20 years younger than me, but now it seems to have worked out OK," the president quipped at a White House ceremony honoring the baseball world champion St. Louis Cardinals.

"I'm 48," the first lady proudly declared to the audience.

ABC's Ann Compton, Mary Bruce, Sara Just and George Sanchez contributed to this report