The event drew 13,000 re-enactors, including 350 cavalry and 100 artillery pieces, said Dennis E. Frye, co-chairman of the Antietam Commemoration Committee. Re-enactors were still arriving Saturday evening.
People from every U.S. state and 15 foreign countries were represented, he said.
"I just marvel at this," said Frye, also president of the Association for the Preservation of Civil War Sites, the event's producer.
All morning, re-enactors marched in formation and conducted drills and demonstrations for the growing crowds.
Rows of soldiers five deep snaked across the 612-acre Artz farm south of Hagerstown.
"Somebody said it was like a trip to the Twilight Zone," said Randy Resh of Grantsville, Md., who was accompanied by nine Boy Scouts.
Former Maryland Gov. William Donald Schaefer said he was moved by the spectacle.
"I can almost hear the shots being fired. I see the men running back and forth. I can see them running over the bodies. I can see the blood," Schaefer said.
The weather, partly sunny and in the 80s, was a little warmer than it was on Sept. 17, 1862, the day of the actual battle, Frye said.
Organizers opened the gates an hour early at 8 a.m. when spectators began lining up outside, Frye said. The air was hazy with smoke from campfires, over which re-enactors cooked their breakfasts.
It was the first re-enactment for Regis and Marian McNally of Pittsburgh, who brought their two young grandsons.
"I really like history a lot and the Civil War is my favorite war the U.S. has ever fought," said Andrew McNally, 9.
While McNally and his younger brother Brendan, 6, watched the re-enactors, other young boys staged mock skirmishes with cap guns.
Under one of the thousands of white canvas tents, George Fickett, 48, called his wife in Prince George, Va., on a cellular phone. Sprint had set up the tent to allow re-enactors to make free personal calls.
Logistical problems came to a head about five minutes after troops began re-enacting Confederate Maj. Gen. A.P. Hill's attack at 3:15 p.m., when Maryland State Police had to begin turning away new arrivals.
The spectator parking lots on both sides of Rench Road were filled with an estimated 15,000 cars.
Before Saturday, the largest tourism event in Washington County was the July 6, 1991, Independence Day concert by the Maryland Symphony Orchestra. That concert at Antietam National Battlefield drew an estimated 35,000 spectators.
Emergency crews treated about 90 people on Friday, mostly for dehydration and exhaustion, said Doug DeHaven, deputy fire chief of the Halfway Volunteer Fire Company.
By 6:30 p.m. Saturday, emergency crews had treated another 100-plus people, according to Alan Matheny, emergency services coordinator for the Halfway Volunteer Ambulance Company.
The most serious problems occurred during the afternoon battle re-enactment, when one person had a heart attack and another was stricken with severe heat-related problems, Matheny said. Both were taken to Washington County Hospital, but their conditions were not available Saturday night.
Another eight people were taken to Washington County Hospital Saturday after experiencing medical problems that included seizures, an asthma attack and a dislocated knee, among other things.
Staff Writers Guy Fletcher and Kerry Lynn Fraley contributed to this story.