“As an agency, we have to prepare at the highest level,” said Malcolm D. Wiley Sr., a spokesman for the Secret Service, which has the overall responsibility for inaugural security. “We understand this is a historic event, but it is also tied to the size of the crowd we expect and our security information.”

The 240,000 people with tickets to the swearing-in and the thousands of additional people with tickets to watch the parade are only a fraction of the numbers expected to walk or take buses or subways to the Mall. Most of those without tickets, if they see anything at all, will watch on 20 Jumbotrons that the authorities, fearing that even a small incident could provoke a stampede, hope will keep people in one place.

While the federal security officials have not projected their total cost for the inauguration, officials in the District of Columbia have said the city might spend nearly $50 million. State officials in Maryland and Virginia have estimated they might spend $12 million and $16 million respectively, and officials in each of the three jurisdictions have said they hope the federal government will help pay for their expenses.

The security plan covers not just the events on Jan. 20, but also the welcome concert for Mr. Obama on Jan. 18 at the Lincoln Memorial, which is expected to draw as many as half a million people, and nearly 100 official balls, concerts and receptions that will not end until the early morning hours of Jan. 21.

So far, analysis of intelligence has not yielded evidence of an overseas or domestic threat, said Joseph Persichini Jr., who is in charge of the F.B.I.’s Washington office. But, he said, the assessment changes daily.