US Secret Service and police tightened a security clamp on Washington on Monday, closing major traffic arteries and throwing up a rigid downtown cordon on the eve of Barack Obama's historic inauguration.
As president-elect Obama criss-crossed the city in his new armoured car to engage in community service events, authorities began choreographing a well-rehearsed plan to turn a three-square-kilometre area of the capital into a hyper-secure zone.
"The closures have begun, and everything is going along as planned," a spokeswoman for the Secret Service's Joint Information Center told AFP less than 24 hours before Obama is sworn in.
Police blocked off streets to all but official vehicles, and National Guard reservists fanned out to take up positions across downtown, where as many as two million people could arrive to witness history on Tuesday.
Military and police helicopters were seen making routine flights over the city, and Secret Service sharpshooters were expected to take up key positions atop government buildings, museums, hotels and offices.
Downtown streets were to fall eerily silent overnight, with access to the district severely restricted from 2.00am when the state of Virginia shuts bridges into Washington and reroutes public traffic around the city until at least Tuesday night.
"The bridge closures are new to this inauguration," Washington department of transportation official Karyn Le Blanc said, noting that Tuesday will see the first US presidential transition since the attacks of September 11, 2001.
"And the fact that it's the inauguration of the first African-American [president] leads us to believe that the crowds will be fairly large," she said.
More than 12,500 active troops and military reservists, thousands of metropolitan police as well as personnel from 57 departments around the nation are descending on Washington to ensure security.
Officials say there is no specific threat on the inauguration, but the Department of Homeland Security has designated it a national special security event.
To counter any potential terror threat, officials are putting in place what is likely to be the biggest and most advanced inaugural security operation ever.
The US military will fly air patrols, man surface-to-air weapons systems, ply the Potomac River with gunboats, assess chemical and biological threats, organise large-scale medical support in the event of an attack, and provide visible and undercover on-the-ground security.
After the inauguration, Obama will be escorted from the US Capitol down Pennsylvania Avenue to the White House in the 56th Inaugural Parade, during which he may choose to walk part of the route or ride in his new limousine – believed to be the safest presidential wheels ever.
Four officers with a Maryland National Guard infantry unit that saw service in Iraq surveyed the crowds on the mall just after sundown.
Major Doug Reed however did not expect trouble.
"Port-a-potties are going to be the biggest security challenge," quipped Reed, referring to the estimated 5,000 portable toilets set up along the mall and near the parade route.
"Our main mission is to provide a presence, to keep people calm," added Major George Downey, with the same guard unit.
Miles of black iron security fencing were laid out Tuesday along the parade route and much of the National Mall, the stretch of grass between the US Capitol and the Washington Monument where masses will gather to witness Obama's swearing-in – after passing through security screening.
They will also be barred from bringing in a variety of items including backpacks, umbrellas, sign supports, coolers, pets, aerosols, bicycles, and any kinds of weapons.
The intense security operation did not deter tens of thousands of US and international visitors from descending on the National Mall on Monday afternoon, although the Secret Service and Washington police have stressed that all members of the public will be herded out of the area on Monday night to allow agents to conduct security sweeps.
One official who will miss out on the celebrations is US Defence Secretary Robert Gates, who will remain at an undisclosed location as the "designated successor" in the event of a catastrophe, the White House announced.