Security staff are seen near the US Capitol building in Washington, D.C., the United States, Jan. 17, 2021. (Xinhua/Liu Jie)

The entire US Capitol complex was on lockdown Monday morning after a fire was reported several blocks away, Capitol Police said in a letter to lawmakers.

"Due to an external security threat under the bridge on I-295 at First and F Streets SE, staff and other personnel are directed to avoid coming to the Capitol Complex area until further notice," read the alert. "All personnel currently on the Capitol Complex are advised to stay indoors and away from windows and doors."

D.C. Fire and Emergency Medical Services said on Twitter that the fire had been extinguished. It said the fire was caused by homeless person who used propane, adding the incident resulted in one individual sustaining "non life threatening injury."

On-site photos showed people inside the Capitol building were evacuated from the West Front of the Capitol. They were rehearsing the 59th Presidential Inauguration slated for Wednesday, when Democrats Joe Biden and Kamala Harris will be sworn in as president and vice president, respectively.

Police ramped up security measures on the Capitol Grounds in the lead-up to the inauguration, as they braced for possible violence after pro-Trump demonstrators stormed the Capitol building on January 6.

Roughly 25,000 National Guard members have been deployed in the nation's capital, with 7-foot barriers being erected around the Capitol, its office buildings and the Supreme Court. Military vehicles were seen parking at intersections downtown to boost security.