"I'm having the time of my life," said Richard Gilby, a 15-year-old Boy Scout from Staten Island, N.Y. "This just moves you. It's history in the making. They only have these every four years, and so far it's been terrific."

Gilby was one of an estimated 32,000 scouts and adult leaders from the United States and around the world who descended today on the Fort A.P. Hill Army base near here for the Boy Scouts of America's 11th jamboree, the largest such event ever held.

The jamboree, which celebrates the Boy Scouts' 75th anniversary, began with breakfast fireworks, the raising of 75 flags and two huge cannon blasts. It was scheduled to continue tonight with a host of activities, including a Beach Boys concert.

This event, two years in the preparation, has trans- formed 7,000 acres of the military base into a city for the scouts, as well as for the 100,000 visitors expected to attend, including Nancy Reagan at the closing program Tuesday night.

It is, at the moment, the 15th largest city in Virginia, with its own postal service, a daily newspaper, a hospital and food warehouses.

Logistically, it is staggering. While they are here, the organization says, the scouts are expected to devour enough eggs that it would take 100 hens 4 1/2 years of steady production to fill the need. They will drink an estimated 300,000 pints of milk, as well as 11,000 gallons of fruit juice and vast amounts of soft drinks and tea.

They also will consume 7 1/2 tons of sirloin steak, 6 tons of bacon, 1 1/2 tons of salami and a like amount of bologna, and three miles of link sausage. They will eat 10 tons of breakfast cereal, 6 tons of granola bars, 5 tons of raisins, 15 tons of dried fruit and nuts, and 100 gallons of mustard.

"Boy, this is just great," said Philip Foster, 16, of Key West, Fla. "It's really the first time in my life that I've ever been in a big event like this. I just want to meet all the foreign scouts I can. I met some English and Scottish ones already, and one of the English ones and I are going to trade uniforms when it's over."

"It's a lot of fun," said David Forrest, 16, of Springfield, Mo., who toured Gettysburg, Pa., and Washington before coming here. His friend, Jerry Nutt, 13, also from Springfield, agreed. "I just want to meet a lot of people," he said.

The program of the 1985 jamboree includes such activities as volleyball, tug of war, computer skills, fire building, flag raising and patrol flag design. The scouts also will participate in archery, an electronic fox hunt, rubber rafting, canoeing, trap shooting, bicycle motocross and other events.

Scout officials said they hoped to play down negative publicity that followed their 1981 jamboree here, when it was discovered that the boys had been camping within 150 feet of soil contaminated with dioxin, a highly toxic herbicide.

There have been no reported illnesses stemming from the jamboree and scout officials said they are confident there is no danger of contamination at this year's event.

The Environmental Protection Agency removed contaminated soil last winter, and after testing they gave the site a clean bill of health.

"What dioxin?" said a smiling Robert Williams, 41, a Boy Scout spokesman.

The first busloads of scouts arrived several days ago, before the official opening. But hundreds poured onto the base today and the dirt roads were filled with a sea of olive green uniforms, backpacks, hiking sticks and brightly colored merit badges.

Jon McCleary, 13, of Olney Springs, Colo., said he was amazed at the size of the jamboree and at eastern cities in general. He flew into New York's LaGuardia Airport and spent several days touring before coming to Virginia. "The airport that we landed at in New York was bigger than our whole town."

As of midday today there had been few problems. About 6:45 p.m. Monday, a scout using a metal detector had found two nonexplosive mine training devices near Rhodes Camp, an area within the jamboree site.

Officials were notified and personnel from the 57th Explosive Ordnance Detachment of Fort Belvoir were dispatched. As a precaution, about 200 scouts were temporarily moved from the area but nothing of danger to the scouts was found.