When the smoke of mock Revolutionary War battle cleared today, the harsh realities of the 20th century remained.

First, all 3,000 participants in the reenactment of the decisive battle of the Revolutionary War, dressed in their 18th century soldiers' uniforms, were effectively disarmed by a modern contingent of Secret Service agents. Before they were permitted to march on to the battlefield for this morning's ceremonies, attended by President Reagan and French President Francois Mitterrand, they had to surrender their musket flints and black powder.

Then, minutes before the heads of state arrived, a band of 100 women and a few children dressed in colonial costumes to resemble the camp followers of the day made an unscheduled effort to follow the men on to the battlefield. They were intercepted and halted by government agents dressed in dark suits.

"We were told we would be shot if we tried to go on to the field," said an irate Janet Anderson, a pretend camp follower from Ohio. "But remember, Americans were rebels. Rebellion is what our revolution was all about."

Today's rebellion among a small number of the 4,000 volunteers who have been playing revolutionary roles during this four-day reenactment, was little noticed by an estimated crowd of 75,000.

Most of the audience was standing behind park police lines, craning to see the soldiers marching on to the grassy battlefield in uniforms as varied in color as the autumn foliage around them.

After the troops had assembled, Reagan, Mitterrand and Britain's Lord Hailsham spoke from a stand equipped with a bulletproof plastic window one-and-a-half inches thick. Military bands played music of the colonial period and 21 jets streaked overhead.

After the dignitaries left, the 18th century soldiers reassembled to act out the surrender ceremony of Lord Cornwallis and his 7,500 British troops who were trapped by a combined American and French military force on Oct. 19, 1781.

"This is a piece of history. I just hope the children will remember it," said Pat Riely, who was standing with her husband and three small children in a crowd that could see the ceremonial field only by standing on tip-toe. Three-year-old Daniel Riely, perched on his mother's shoulders and waving a small U.S. flag, seemed less interested in the official program than the crowd that gathered for it.

To the right of the Riely family, a half-dozen people held signs protesting America's foreign policy, including one that read "George Washington was a leftist guerrilla." A man in an Uncle Sam costume, sporting a white goatee, distributed mimeographed copies of a song he composed in honor of President Reagan entitled, "Let Ron Do It."

Security was tight today for President Reagan's visit, which came not quite two weeks after Egyptian President Anwar Sadat was assassinated at a military show and two years after a Williamsburg author published a novel, "The Allah Conspiracy," which details an assassination of an American president at the Yorktown Bicentennial by Arab terrorists.

After the 3,000 volunteer soldiers were stripped of their flints and gunpowder, each was subjected to a scan by a portable metal detector. Security was also cited as a reason the women and children were not permitted to follow their troops onto the field until after Reagan and Mitterrand departed.

"History has been pushed to the background," said Carrole Rugenstein, a pretend British camp follower from Michigan, upset that after four nights in a tent and four days in front of an 18th century cook fire she and the other followers were not allowed to take the stage while Reagan and an international television audience were watching.

Soldiers had their own complaints about the way the four-day ceremony was planned. Many were particularly upset with the National Park Service, which has jurisdiction over the battlefield occupied this week by a mock army. Soldiers complained that their battles were shortened, their fire power reduced and, for at least one major reenactment, no one was allowed to die.

"Some of these guys came a long way and never got to fire their guns," said David Strand, a professional photographer from Michigan and a member of a colonial artillery unit that rebelled Thursday night. The artillery soldiers, incensed after the Park Service refused to allow them to use more than six of the 50 cannons they brought for the battle, threatened to leave their cannons by the bleachers and watch the rest of the reenactment from the stands.

After an all-night negotiating session, leaders of the artillery unit backed down on their threat. The controversy left bitter feelings.

"Obviously we're not too popular because we're enforcing regulations that cramp their style," said Roy Graybill, a black powder safety specialist for the Park Service. "But our job is to make them safe, not happy."

Soldiers were successful in getting the army to stop hovering helicopters over the battlefield during 18th century skirmishes.

"We're trying to slide back into an 18th century frame of mind and helicopters kind of wreck that," said Robert Wright, a U.S. Army military historian who was serving as press liaison for the volunteer soldiers.

If the recreationists left this celebration a little unhappy, the 200,000 spectators who flocked here over the four days seemed well pleased. Because the crowds were so large and this town so small, there was much waiting in line. Planners said there were surprisingly few mistakes and a camaraderie developed on many buses stuck in festival traffic.

Today, for example, Ron Springs, a Norfolk carpenter brought his wife and two teen-age children to the commemoration. Standing in the middle of the thick crowd, Springs could see little but the rows of spectators in front of him.

"That's okay," said Springs, who still was happy to have been where history was made. "We can go home and watch it on the news tonight."