Correction: An earlier version of this article incorrectly described Quantico Marine Corps Base as being near Manassas. Quantico is in southeast Prince William County, while Manassas is some 20 miles away, in the northwest corner of the county. This version has been corrected.

As many as 30 protesters were arrested near Quantico Marine Corps Base on Sunday while calling for the release of Bradley Manning, the Army private accused of sharing a large cache of classified military intelligence with the Web site WikiLeaks.

The arrests of the protesters came at the end of a largely peaceful demonstration of about 400 people at Quantico. Among those arrested was Daniel Ellsberg, 72, a former military analyst who became nationally known after releasing the top-secret government documents called the Pentagon Papers to newspapers during the Vietnam War.

Before the arrests, protesters had arranged, as a gesture to Manning, to place flowers at a memorial statue that commemorates the Marines raising the flag at Iwo Jima in World War II. But police tightly choreographed the protesters movements and, in the end, asked them to toss the flowers through a gate at the statue.

“They wouldn’t even let us get up to the memorial,” said former Army colonel and State Department official Ann Wright. Saying she was fed up, Wright sat in the middle of Jefferson Davis Highway, as did Ellsberg and 28 others.

“It was disrespectful,” Wright said of the police.

The impromptu sit-in led to a tense standoff between the demonstrators and Manassas, Prince William County and Virginia state police, who were in riot gear and on horseback, with some carrying automatic assault weapons. They advanced on the squatters and took them away one by one.

“I thought we had to do this, to show we had some fortitude,” said Ann Wilcox, an attorney for the demonstrators. “If we had quietly gone back, we wouldn’t have made the statement that we made.”

The demonstrators had made noise for two hours, shouting “Free Bradley Manning!” and carrying signs denouncing the Obama administration and the military for his treatment in the brig at the base.

Manning, 23, is accused of downloading tens of thousands of secret documents and at least one video onto his computer while stationed with the 2nd Brigade Combat Team, 10th Mountain Division, in Iraq. The video taken by cameras on U.S. helicopters showed a military strike that killed civilians, including two news agency workers.

The soldier was initially charged with two offenses, detained and taken to Kuwait in May. He was eventually brought to Quantico, where officials levied 22 additional charges, one of which, “aiding the enemy,” could bring the death penalty.

A network of supporters sprang up, and Sunday’s demonstration was aimed at expressing dismay at Manning’s treatment. There were at least eight other demonstrations around the world, including in London, Sydney and Berlin.

Manning has been confined to a small cell for 23 hours a day and forced to strip naked for inspection each night after he quipped — innocently, his attorney said — about suicide. Prison psychiatrists have said Manning does not represent a danger to himself.

Ellsberg, who was branded as a traitor for releasing the Pentagon Papers and was later widely embraced by those who opposed the war in Vietnam, said he has supported Manning since his arrest.

“I identify with him more than anyone else I’ve seen in the last 40 years,” Ellsberg said. When he released the Pentagon Papers, he said, “I was willing to go to prison and give my life and be executed.”

Jules Orkin, 72, drove from Bergenfield, N.J., to support Manning. He sat on the edge of Jefferson Davis Highway in prisonlike orange garb and with a black bag over his head to demonstrate his belief that Manning is suffering a form of torture.

“I think he served a higher honor to expose things we’re doing wrong,” said Orkin, who described himself as an Army veteran.

Lisa Cantoni, 43, of Woodbridge said the protests were a disgrace. “We are Americans, and he’s supposed to be on our side,” she said of Manning. “He should be punished. He should not be freed. . . . That’s preposterous.”

As demonstrators marched from a muddy staging area, passengers in a car gestured out the window and shouted, “Traitors!” But far more drivers sounded their horns in support.

A police officer shouted directions to the marchers like a broadway director: “Okay, move to your right. . . . Now people with flowers, move this way. . . . Members of the media, you should step to your left.”

The instructions infuriated the demonstrators.

One protester started the “Free Bradley Manning” chant, and soon the 400 protesters roared back. When Wright, Ellsberg and the other demonstrators sat sown, police lost control and declared the protest unlawful.

The demonstrators were becoming more emboldened when an announcement that their transportation was about to depart quieted much of the protest.

“The buses back to Union Station will leave for Washington in 20 minutes,” said a voice over a megaphone. “They’ll start boarding in five minutes.”