The Capitol Police on Monday alerted people at the Capitol and surrounding complex not to enter or exit for a brief period Monday morning because of what was characterized as “an external security threat,” which turned out to be a small fire several blocks away.

A rehearsal for Wednesday’s inauguration of President-elect Joe Biden and Vice President-elect Kamala D. Harris was underway at the time, with other people standing in for each of them and some of the other principals. Neither Biden nor Harris was present.

On the west side of the Capitol, where the rehearsal had just begun, U.S. Capitol Police ushered participants from the platform and into the Capitol, where some milled about and others sheltered in place in rooms.

Capitol police also knocked on doors of media trailers parked outside the building and shouted, “Go into the Capitol!” as a voice came over a loudspeaker announcing an “external security threat.”

“Due to an external security threat located under the bridge on I-295 at First and F Streets SE, no entry or exit is permitted at this time,” the police said in an alert that went out to members of Congress, staffers and others who work inside the building shortly after 10 a.m. “You may move throughout the buildings but stay away from exterior windows and doors. If you are outside, seek cover.”

From the Capitol, where Biden is scheduled to sworn in at noon Wednesday, smoke could be seen coming from the location of the bridge.

Shortly afterward, D.C. firefighters announced that they had responded “to an outside fire in the 100 block of H St SE that has been extinguished.”

“There were no injuries. This accounts for smoke that many have seen,” D.C. Fire and Emergency Medical Services said on Twitter.

The alert was lifted and the all-clear given shortly after 11 a.m., and the rehearsal resumed.

D.C. police spokesman Stephen Benson said the fire started at a homeless encampment. He said the alert was sent because of the proximity of the fire to the Capitol, which — along with much of the rest of downtown Washington — has been on high alert since the Jan. 6 breach of the Capitol by a mob protesting President Trump’s election defeat.

The fire occurred after propane in a Coleman heater exploded, said D.C. Fire spokesman Vito Maggiolo. A woman said she had set a fire to try to keep warm in her tent and that the fire spread, igniting her tent. At some point, Maggiolo said, propane in the heater caused a “very minor” explosion.

He said the woman had declined to be taken to a hospital for minor injuries but was receiving help from the American Red Cross.

Peter Hermann and Toni Sandys contributed to this report.