BOONSBORO, MD., SEPT. 27 -- In the gray light of early morning, an estimated 9,000 people gathered on a gentle hill in rural Maryland today to cheer the 125th anniversary reenactment of one of the most violent struggles in American history and the bloodiest day of the Civil War -- the Battle of Antietam.

They came with blankets and jugs of hot coffee, children hoisted on their shoulders and ball caps on their heads. From the time the first cannon boomed at dawn until the last exploded more than an hour later in a cloud of black powder and orange sparks, people watched, transfixed.

"Incredible," said Richard Duckas, 24, a painter from Baltimore. "You can't believe it's not real."

This reenactment, and a similar one later in the day, of the battle that killed 23,000 soldiers on Sept. 17, 1862, and stopped Confederate Gen. Robert E. Lee's drive north, topped off a two-day anniversary observance.

Throughout the weekend, Civil War enthusiasts and reenactors gathered on a Boonsboro farm near the Sharpsburg battlefield and talked about their favorite subject: the Civil War.

Whether born of a love of military history, or out of affection of things paramilitary, grown men purchased souvenir forage caps, looked at the actual surgical implements used to perform amputations, marched through cornfields and pretended to be hit by blasts of musketry.

Reenacting: It's a serious business. Soldiers wear reproduction clothing and spend the weekend in reproduction camps, complete with reproduction Civil War camp furniture. Antietam this past weekend was the place to see what Civil War-era shoes looked like, and polished brass buttons, and rifles. A Civil War-era stethoscope, which looked a lot like a car radiator hose, also was available for examination.

That this was play and not real was underscored as rows of soldiers marched past the line of white Sani-Pots, when others retired from the battlefield in a Toyota, and when one soldier called out to another: "I didn't bring any rags to clean my gun; guess I'll have to use some paper towels."

Phil DiMaria, 35, one of the reenactment soldiers, had gone to Antietam with Battery B, the first Rhode Island light artillery. At home, he is a designer for Hasbro, the toy company. DiMaria said his military interests are not unusual among his colleagues; Hasbro, after all, is the company that produces GI Joe, and DiMaria has just finished work on a space shuttle toy.

Another New Englander at the reenactment was Vincent Cantoni, 38, owner of a Plymouth, Mass., coal and oil firm. He has long been interested in the Civil War but decided only six years ago to take up a gun and participate in reenactments. "I have yet to find myself bored," he said.

Also there, wearing a long Civil War-era dress, was Barbara Laird Hall, 33, of West Warwick, R.I., whose great-grandfather fought at Antietam. From a trunk inside her tent, she pulled out yellow photographs of her grandfather, who she said was born in 1840 and lived until 1922.

The festivities were organized by the American Civil War Commemorative Committee, which reenacted the first Battle of Bull Run last summer, said development director Juanita Mary Leisch.

A sour note: Crowd control was so poor at the dawn reenactment that many people were unable to see much except the tops of the flags moving through the cornfield.

It was not immediately certain how much money was raised, but proceeds from ticket sales and parking were to help finance the event and to benefit the Save Historic Antietam Foundation, a group working to preserve the battlefield from nearby development.

Also there during the weekend -- and working hard to collect petition signatures -- was Ed Wenzel of Fairfax County's Chantilly Battlefield Association.

His nonprofit volunteer group hopes to persuade the Fairfax County Board of Supervisors to acquire 2.5 acres of the county's only significant battlefield, most of which has been swallowed by the Fair Oaks Mall shopping center.

Echoed Brian Pohanka of Alexandria, a Civil War historian and reenactor who attended the weekend's festivities: "Even while we celebrate the 125th reenactment of Antietam and stage these epic reenactments that thousands can enjoy, Civil War sites are being lost, and it's a real tragedy."