As multiple groups were meeting to practice for the festivities on Wednesday, smoke was seen rising up behind the Capitol Building ... forcing an evacuation.

According to reports ... Capitol Hill staffers received a message warning of an "external security threat" prior to the dress rehearsal for the inauguration.

Everyone was ordered back inside and instructed to stay away from windows and doors and seek cover if outside.

The good news -- the D.C. Fire Dept. located the fire and put it out ... and nobody was hurt. Our sources say the fire was started at a homeless encampment and is no longer viewed as any sort of threat.

Of course, there is a very strong security presence everywhere in the area -- bolstered by some 20,000 National Guard troops -- and the brief evacuation is a sign of the high alert that remains since the Jan. 6 Capitol riots.