The Army is activating the entire District of Columbia National Guard — 1,100 troops — in response to a request from Mayor Muriel Bowser of Washington, an Army official said on Wednesday.
The troops are being sent to the D.C. Armory and will be deployed to the Capitol and to other points around Washington, the official said.
In a statement, Jonathan Hoffman, the chief Pentagon spokesman, said the Guard had been mobilized and the response would be led by the Justice Department. The decision by Army Secretary Ryan McCarthy and Christopher C. Miller, the acting defense secretary, came as protesters breached the Capitol.
Mr. Miller said on Wednesday afternoon that he had spoken with Vice President Mike Pence, Speaker Nancy Pelosi, Senators Mitch McConnell and Chuck Schumer, and Representative Steny H. Hoyer of Maryland about the protests at the Capitol.
“We have fully activated the D.C. National Guard to assist federal and local law enforcement as they work to peacefully address the situation,” Mr. Miller said. “We are prepared to provide additional support as necessary and appropriate as requested by local authorities. Our people are sworn to defend the Constitution and our democratic form of government, and they will act accordingly.”
Mr. Hoffman noted that the troops were deploying in “support” of federal law enforcement in the district, reflecting Defense officials’ reluctance to send military troops to the Capitol. Defense officials want the authorities in Washington to use the local police and other law enforcement agencies to confront the protesters, with the National Guard troops in support but not in the lead, to avoid the specter of a military battling election protests.
But the tense standoff at the Capitol, and the breach by Trump supporters, led to the decision, officials said.
The F.B.I. also mobilized its officers to reinforce the Capitol Police, a spokesman said.
Bureau agents went to the Capitol grounds on Wednesday to help the police on the scene protect the building and the public. A handful of the F.B.I. agents arrived in camouflage and bearing shields and machine guns late in the afternoon outside the secure location where the senators were being held.
The deployment signaled the growing alarm among federal officials viewed the chaos swirling around the Capitol building on Wednesday.
“The F.B.I. has been deployed to assist our U.S. Capitol Police partners, as requested, in protection of federal property and public safety,” the F.B.I. statement said.
The bureau would not say how many of its agents were deployed to the capitol. But a former government official said that earlier on Wednesday, all F.B.I. agents in the region were alerted that they could be called to downtown Washington.