By Arshad Mohammed and Michelle Nichols

WASHINGTON (Reuters) - Police declared an all-clear at Washington's Union Station and allowed people to re-enter the train terminal after a bomb threat on Wednesday forced a brief evacuation.

"We didn't find nothing. We wouldn't let you back in if it wasn't all clear," said a police officer, who did not give his name.

An Amtrak employee described the incident as a "false alarm" and said there would be a slight delay for trains.

The entire incident lasted less than an hour.

Scores of panicked people ran out of the U.S. capital's main railway station while police investigated a possible suspicious package after an earlier report of a bomb threat, police said.

A Reuters witness said people began running out of the station, a hub for railway, commuter train and subway service in the city, shortly after 5 p.m. EDT (2100 GMT).

During the alert, at least 20 police cars, a fire truck, an ambulance and vehicles from the U.S. Department of Homeland Security's Federal Protective Service converged on the station.

(Reporting by Arshad Mohammed and Michelle Nichols; Additional reporting by Richard Cowan; Editing by Eric Beech and Sandra Maler)