217 people were arrested and six police officers suffered minor injuries after some protesters set fires and smashed windows in the nation's capital.
The operation, especially in the post-9/11 era, is the most sophisticated protection and communications system in the world.
"As you looked into his eyes you could see the surprise, you could see the shock, and ...you could actually see the fear," a federal agent said.
Watch the second half hour of the Nightly News broadcast on the day of the Presidential Inauguration on January 20.
Hundreds of jobs have not yet been filled by Trump appointees.