As the Secret Service prepares to shut downtown Washington days earlier than first planned, the acting police chief in Washington, Robert J. Contee III, announced Wednesday that an additional 5,000 National Guard troops would be deployed to the city to support local law enforcement providing security for Mr. Biden’s inauguration, bringing the total number of Guard personnel to 20,000.

That is roughly three times the total number of American troops deployed in Iraq, Afghanistan, Somalia and Syria.

More than 3,000 National Guard troops, rotating in 12-hour shifts, will provide security in and around the Capitol at any given time. Other troops will help control traffic and Metro stations elsewhere in the city. Members of the Guard at the Capitol will be equipped with M9 sidearms and some will carry automatic rifles and shotguns. All will have protective riot gear, including helmets and Kevlar vests, Guard officials said.