Bridges into Washington and about 3.5 square miles of downtown will be closed Tuesday. The security perimeter covers more of the city than in previous inaugurations.
Thousands of extra police, military troops and law enforcement agents, including plain clothes officers roaming the crowds, will be on hand to handle the potentially 2 million people who could descend on the nation's capital.
People attending the ceremony and parade can expect to be searched by machines, security personnel or both. Precautions will range from the routine - magnetometers, like those used at airports - to countersnipers trained to hit a target the size of a teacup saucer from 1,000 yards away.
There will be undercover officers, bomb sniffing dogs and air patrols. Washington's 5,265 surveillance cameras, spread around the city, are expected to be fed into a multi-agency command center. Including the Secret Service, 58 federal, state and local law enforcement agencies are providing security.
"We're nearing the event and all of the planning that we've been doing for all this time is starting to come together," Secret Service spokesman Malcolm Wiley said.
The agency is the overall lead for security, but issues and operational decisions are handled collaboratively by all involved, Wiley said.
Security commanders will be a central command center. As different situations arise, they will talk to each other about how to address them. If a decision is made, for instance, to use tear gas, the commander of the jurisdiction of the incident will give instructions. So if the incident is on the National Mall, it would be the Park Police communicating the orders to the officers in the field.
Chertoff, whose department includes the Secret Service, will not leave his position until the day after Obama is sworn in. Chertoff will be the only member of President Bush's Cabinet - except Defense Secretary Robert Gates, whom Obama asked to keep that job - to remain in his position after Bush leaves office.
Thousands of officers from 40 police jurisdictions will line the 137-mile route from Philadelphia to Washington on Saturday. Crowds are expected to gather at numerous spots, including overpasses, parking lots and commuter train stations. Obama is retracing the journey of Abraham Lincoln, who also rode to his inauguration on a train from Philadelphia. Lincoln was smuggled under cover of darkness from one train station to another to avoid a feared assassination attempt.
The Metropolitan Police Department's 4,100 officers and an additional 4,064 officers from police departments across the country will be on duty, said John Cohen, a senior adviser in the terrorism information sharing office of the director of national intelligence. Cohen said they have all completed a special online training course to help them detect and report suspicious activity that could be related to terrorism, such as photographing restricted sites, collecting information about secure areas and stealing official uniforms.
The city has also trained some employees in the public works department, said Patrick Burke, the police department's assistant chief for homeland security. "The purpose is to get as many people collecting suspicious activity reporting as possible," Burke said.
The FBI has started a system to share tips about possible terrorist threats with local police agencies.
For the first time, the president declared an emergency in the District of Columbia in advance of the inauguration. That will cover 100 percent of eligible local inauguration costs for emergency protective measures on Tuesday. It is in addition to the $15 million Congress set aside for Washington inauguration security costs.
The large crowds predicted for the events raise logistical, security and public safety concerns.
"It only takes a minor event to create a stampede of people," said Wendell Shingler, former director of the Federal Protective Service, which provides security around government buildings.