WASHINGTON — A city already well-versed in pomp, circumstance and crowd control is in serious overdrive this week with just 13 days to go until millions of people descend upon the U.S. capital for Barack Obama’s inauguration.

Almost as soon as the ballots had been counted after the Nov. 4 presidential election, D.C. moved into hard-core preparation mode for both Obama’s historic Jan. 20 swearing-in and the weekend events leading up to it.

The city has been abuzz with workmen constructing barricades and bleachers on the parade route along Pennsylvania Avenue from the Capitol Building to the White House.

They were still busy Tuesday working on huge banks of bleachers on the western lawn of the majestic Capitol, where Obama will make his inaugural address overlooking both a sea of spectators and some of the world’s most famous political landmarks.

Security officials are also working overtime this week sorting out logistics for what’s believed to be the largest security operation ever for an inauguration. Thousands of extra police officers, federal agents and troops will be on hand when Obama is sworn into office as the country’s 44th president.

With an estimated four million people expected to show up on what could be a cold winter’s day — meaning they’ll be bundled up in heavy clothing — security officials concede the challenges are immense.

“We expect a large crowd and one of the biggest that we’ve ever had to deal with,” Sgt. Robert Lachance of the U.S. Park Police said in an interview Tuesday.

Due to the sheer magnitude of the crowd, Lachance said it would be impossible to frisk everyone in attendance and to check their bags and knapsacks.

“We’re not going to have traditional security checkpoints but we are going to have a large amount of officers highly visible to the crowds, and we’ll have plainclothes people among the crowds keeping a close eye on what’s going on,” he said.

“There will be a lot of security that people will never even see and will never even know is going on, due to us having so many plainclothes officers in the crowds and other measures that we can’t divulge. But the welcome mat is out, and we want people to be able to take part in this inauguration.”

There will also be mounted police officers surveying the crowds and cameras mounted in countless elevated positions along the parade route.

Security officials aren’t just preparing for the swearing-in ceremony — there will be as many as 70 concerts, balls and other events surrounding the inauguration.

The Secret Service is overseeing the inaugural security plan, working with 57 other federal and local agencies. Twenty-three subcommittees are focusing on issues ranging from explosives to civil disturbances.

The FBI has also reportedly been leaning on its intelligence networks to determine if any any suspected extremists might be headed to the U.S. for the inauguration. So far there’s been no threats.