Members of the US National Guard have been deployed to the US Capitol building.
Some were seen taking pictures of the monuments and taking a break on the steps that were occupied by rioters last week.
Another 10,000 will arrive on Saturday as the FBI warns of the risk of more violence from armed extremists holding pro-Trump events in 50 state capitols between now and inauguration next week.
Washington DC Mayor Muriel Bowser is urging people to avoid coming to the city for Joe Biden's presidential inauguration on 20 January.
She says people should participate virtually instead.
At a news conference on Monday, Bowser said her goal was “to protect the District of Columbia from a repeat of the violent insurrection experienced at the Capitol and its grounds”.
In recent days, she has called for federal agencies to beef up security preparations for the ceremony. She also said President Trump must declare a pre-emergency declaration so the city can coordinate resources with the government.
Activists have also taken to social media to urge rental property owners not to list their homes in the days leading up to the event.