Washington DC – The US Capitol was briefly locked down Monday after a fire broke out a few blocks away in the middle of a rehearsal for President-elect Joe Biden’s inauguration, putting security precautions through an early stress test in the wake of this month’s bloody pro-Trump attack on the building.

Participants, including members of a military band practicing The Star-Spangled Banner, were quickly whisked away by Capitol Police officers and Secret Service agents after a thick plume of black smoke shot up in the air about five blocks away around 10:30 AM local time.

"All buildings within the Capitol Complex: External security threat, no entry or exit is permitted, stay away from exterior windows, doors. If outside, seek cover," read a message sent to congressional staff during the evacuation.

But within 40 minutes, law enforcement and emergency officials were able to determine that the fire was unrelated to the inaugural preparations.

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The blaze apparently started in a homeless encampment under a nearby highway overpass, and the city’s fire department said no injuries were reported.

The cause of the fire was not immediately known, but security officers lifted the lockdown before noon, allowing the rehearsal to resume.

The US Secret Service, which is leading security operations for Biden’s Wednesday inauguration, said it ordered the fire-related evacuation "out of an abundance of caution."

"There is no threat to the public," the agency said in a statement.

Neither Biden nor Vice President-elect Kamala Harris were at the Capitol when the fire erupted.

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Biden and his wife, Jill Biden, were in Philadelphia marking Martin Luther King Jr Day by volunteering at the headquarters for a hunger relief organization in the city.

Wearing black face masks, the Bidens helped pack cans of food into boxes on a conveyor belt.

Harris and her husband, Doug Emhoff, spent the day volunteering at another food relief organization in Washington.

"Today in America, 1 in 6 families is describing their household as being hungry, 1 in 5 is describing an inability to pay rent, 1 in 3 is describing an inability to pay their bills," Harris told reporters at the event.

"So we are here today as part of what we collectively all of us who are volunteering see as our responsibility as part of Dr. King’s legacy. And we’re here to renew the commitment that we have to service and to serve others, especially those in need."

President Donald Trump, who has refused to take responsibility for instigating the January 6 riot at the Capitol, stayed out of public sight Monday and did not offer a statement on the federal holiday.

The precautionary shutdown of the Capitol comes at a fraught time.

Tens of thousands of National Guard troops have been dispatched to Washington ahead of Biden’s inauguration amid warnings from the FBI that armed far-right groups may attempt to pull off another attack, with the city still reeling from the January 6 assault on the Capitol that left five people dead, including a police officer.

In an alarming development, Pentagon officials, concerned that some Guard members may be adherents of far-right ideologies, pushed the FBI over the weekend to vet all 25,000 troops expected to enter the city.

Meantime, the National Mall, usually a place for supporters of the new president to gather during the inauguration, has been completely sealed off at least through Thursday, and security personnel erected large steel barriers around Capitol Hill over the weekend, making Washington look like a city at war.