WASHINGTON (AP) — This year’s inauguration is expected to set many records — from the largest crowds, to most bridges closed to the highest security of any president’s swearing-in.

The inauguration is considered a National Special Security Event, a federal designation that puts the Secret Service in charge of security for the entire day. But this year the special security tag has been extended to four days, starting when President-elect Barack Obama starts making his way to Washington from Philadelphia on Amtrak this Saturday.

“It will be the most security, as far as I’m aware, that any inauguration’s had,” Homeland Security Secretary Michael Chertoff said in a recent interview with The Associated Press.

Intelligence officials say there are no specific threats to the inauguration, although the high visibility of the event, the presence of dignitaries and the significance of swearing in the first black president make it a vulnerable target.

Bridges into Washington and about 3.5 square miles of downtown will be closed Tuesday. The security perimeter covers more of the city than 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 as well as undercover officers, bomb sniffing dogs and air patrols. And 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,” said Secret Service Spokesman Malcolm Wiley.

The Secret Service is the overall lead for security, but issues and operational decisions are handled collaboratively by all the agencies involved in the effort, Wiley said. The commanders of all the different agencies involved in security on Inauguration Day will be in the multi-agency command center. As different situations arise, the commanders will talk to each other about how to address it. 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 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 as president, making him the only member of President Bush’s Cabinet — save Defense Secretary Robert Gates whom Obama asked to stay on — to remain in his position after Bush leaves office. Even if his successor, Arizona Gov. Janet Napolitano, is confirmed by Tuesday, Chertoff will remain at the helm to oversee the inauguration, with plans to officially resign at 9 a.m. EST on Wednesday.

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, as Obama retraces 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. And 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 FBI also has launched a system to share tips about possible terror 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. This 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. Shingler, who participated in the 2005 inauguration security plans and operations.