Somewhere within that seemingly boundless sea of empty folding chairs on the Mall was the one meant for Barney O'Hare.

He and about 150,000 others are expected to fill those chairs for the dedication of the National World War II Memorial today. But surveying the fenced-off seating areas yesterday afternoon, O'Hare couldn't guess which chair he'd end up in. He was able, however, to imagine what the view from that seat might look like.

"Crowded," predicted the 77-year-old Army veteran from Jeffersonville, Ind. "Heck, it's crowded today."

Thousands of veterans had the same idea as O'Hare yesterday, as they teemed around the seating areas for a glimpse of what might await them this afternoon. They scanned lists of prohibited items -- food, glass containers, coolers, aerosols and so on. They traced shuttle bus routes on maps. They made mental note of nearby portable restrooms. They tested the plastic runners that had been laid out on the grass to make walking -- or rolling, in the case of many -- easier.

After a 17-year preamble and months of detailed planning, today is what the fuss has been all about: the star-spangled pageantry, the security, the emergency medical tents and all else that accompanies the country's largest organized gathering of World War II veterans in 60 years. Today marks the culmination of an effort that began in 1987, when legislation was introduced to create the $175 million memorial.

"It's such a huge operation," said Doug Moyer, a contractor for the U.S. Postal Service who was among the many workers yesterday rehearsing their roles for the ceremony; his role is to help unveil a postage stamp marking the dedication. "There are a lot of people working very hard on this, and I'm sure it'll be a late night."

Following last night's rush hour, officials shut down 17th Street between Constitution and Independence avenues, closing much of the Mall to vehicular traffic for the holiday weekend. Security guards lined the fences that surrounded the ceremony's main stage near 17th Street, and helicopters ran regular routes overhead. Armed guards monitored nearby Metro stations. The U.S. Coast Guard and D.C. Harbor Patrol prepared to guard the Potomac River, and U.S. Capitol Police officers expanded their program of random vehicle checks.

With a guest list that includes President Bush, presumed Democratic presidential nominee John F. Kerry and former presidents Bill Clinton and George H.W. Bush, security is at the top of the organizers' list of priorities. And with an audience that includes many in fragile health, the logistics are more complicated than they would be for other large-scale events.

Dwight Pettiford, acting chief of the U.S. Park Police, strolled around the grounds yesterday and found himself inundated with questions from veterans. They wanted to double-check where they should be, how they should get there and what time they should arrive. Pettiford told them that seating areas would open at 8 a.m.; many of the veterans assured him they planned to be there bright and early to get a jump on the crowds.

That drew a chuckle from Pettiford. Because all attendees seated in sections nearest the stage will pass through screening stations and metal detectors, he had considered advising people to come early to allow time for the security checks. But after talking to the veterans scoping out the scene yesterday, Pettiford decided to hold off on that advice.

"Would I recommend coming early?" Pettiford asked with a laugh. "I don't think I'm going to have to."

Organizers are concerned that if attendees arrive several hours ahead of the 2 p.m. ceremony, the risk for health problems increases, particularly if the sun is out. At the dedication of the Korean War Veterans Memorial in 1995, 93-degree temperatures resulted in one cardiac arrest and 500 cases of heat exhaustion. Today is expected to be considerably milder -- with highs in the mid-70s -- but the crowd is also expected to be considerably older.

Officials are encouraging ticketed attendees to take shuttle buses to get to the three seating sections for the event. The shuttles will run from the Metro Center, Smithsonian, Farragut West and L'Enfant Plaza Metro stations.

Those without tickets can watch the ceremony on large-screen monitors at two locations on the Mall -- between 14th and 10th streets and on the U.S. Capitol's West Lawn. Non-ticketed attendees, who will not be allowed on the shuttle buses, are being encouraged to walk to those areas from nearby Metro stations, such as Smithsonian and Capitol South.

Free bottled water will be available throughout the Mall, and nine medical tents have been erected -- three between the Lincoln Memorial and the World War II Memorial; two between 17th Street and the Washington Monument grounds; two between 14th and Seventh streets; and two between Seventh and Third streets. Each tent will be staffed with a doctor, two nurses, a medically trained park ranger and nine emergency medical technicians. Six of the EMTs will roam the crowds near their tents. Each tent will be stocked with medical supplies, defibrillators and cots and will include telephone lines in case cellular telephone signals and radio signals are jammed.

Also, nine basic life-support ambulances and four advanced life-support ambulances will be on hand. Medical volunteers will be stationed at bus stops, and National Park Service and D.C. EMTs will circulate through the area on bicycles. A medevac helicopter also will be stationed near the Mall.

Yesterday and Thursday, the first two days of a four-day World War II-themed exposition on the Mall, the extensive emergency preparations went largely untapped, according to D.C. officials. No medical emergencies were reported on Thursday, and yesterday's complications were minor.

"It's been windy, and there is some dust blowing around," said Alan Etter, a spokesman for the D.C. Fire and Emergency Medical Services Department. "We've had to irrigate a few sets of eyes, but nothing at all major."

World War II veteran Francis W. Nye, 86, right, greets Korean War vet Andrew Anderson, 67, and Bernice Crockett, 75.Park Service SWAT team members watch over the new memorial from the stage where the dedication will take place under tight security this afternoon. Wilfred E. Billey, 80, greets fellow ex-Marine Ward Eysaman, 57. Eysaman is a Vietnam veteran, and Billey was a Navajo code talker during World War II.