By Jonathan Beale
BBC News, Washington
There was a time when a US president could travel from his inauguration in an open-top car.
Secret Service agents will mingle with the crowds
John F Kennedy was the last president to do that.
He was also the last president to be sworn into office without the protection of a bullet proof screen.
Those days have long gone.
Secret Service agents now swarm a new president's blast-proof limousine as it travels along Pennsylvania Avenue.
But the inauguration still presents a security nightmare.
And Barack Obama's inauguration is the biggest challenge yet.
There have been painstaking preparations, with rehearsals for security officials - not just for the military precision of the ceremony and the parade.
At a US military base in Washington they have been planning for any eventuality for the past six months.
Huge satellite maps of the centre of Washington dominate the control room at Fort McNair.
On the day itself they will be able to track the new president's every move - and the huge crowds expected to watch.
They are hoping for the best, but preparing for the worst.
Major General Richard Rowe takes me through the possible scenarios: a major power failure, a car bomb or multiple ones, a cyber attack, the collapse of a bridge, and panic among the crowd.
He says his biggest fear would be a biological, chemical or radioactive attack.
But they are ready for anything.
Outside the control room, members of the US Coast Guard show off the latest sensors to detect such a device.
This is only the second inauguration to take place since the attacks on 11 September 2001.
And Major General Rowe reminds me that America is still at war.
In the skies above Washington there is already a sober reminder - helicopters on patrol, joined by fighter jets.
If you are hoping for a glimpse of the new president, get ready for suffocating security and a long wait in the cold
Along Washington's waterways the coast guard is watching from high speed boats.
Snipers are positioned on the rooftops near the Capitol building and along the parade route.
Roads and bridges into Washington are closed to traffic, with sniffer dogs on the subway and thousands of armed police, soldiers and plainclothes agents on the streets.
In total, there are 4,000 Washington police, reinforced by another 4,000 officers from all over the country.
Thousands more National Guard members have been called in.
That is on top of the 5,000 professional soldiers and sailors on the ground.
The new presidential limousine will get its first outing
There are 57 different government agencies involved in what has been declared a "National Special Security Event".
And overseeing the entire operation is the US Secret Service.
It describes itself as "one of the most elite law enforcement organisations in the world", and has had the task of protecting the president ever since William McKinley's assassination in 1901.
Its most visible agents are the ones who will be running beside the presidential car.
But there are also hundreds of others mingling in the crowd.
The Secret Service has already been shadowing Barack Obama for a year.
But since his victory it has been stepping up his protection.
It has also ordered a new presidential limousine.
Dubbed "the beast", it looks more like a tank than a car.
So far it has only been seen in photos, but inauguration day gives the public a chance to see it for real.
When I asked Ed Donovan - the assistant special agent in charge - about the car's features, all he could say was that: "It's made by Cadillac".
Clearly, it is called the Secret Service for a reason.
Agent Donovan says his organisation recognises the "historical significance" of protecting the first African-American president.
It too has rehearsed for any eventuality - whether it is a lone sniper or a terrorist attack.
Preparations for this day have been going on for more than a year, with officials looking at every detail right down to the credentials needed to enter the dozens of inauguration balls.
And then there are the crowds.
Nobody is sure exactly how many people will have turned up - estimates vary wildly from the hundreds of thousands up to four million.
Anyone hoping to get near the parade or the swearing-in ceremony has to go through a security screen, while umbrellas, pushchairs and large banners are among the long list of items that have been banned.
Inauguration Day 2009 may well be the biggest show on earth.
But if you are hoping for a glimpse of the new president, get ready for suffocating security and a long wait in the cold.