Washington (CNN) -- Hundreds of protesters gathered outside the gates of Quantico Marine Base in Virginia Sunday to protest the treatment of Bradley Manning, who is being held at the base prison on charges that he released classified government documents to WikiLeaks.

Daniel Ellsberg, the 1971 Pentagon Papers leaker and Retired U.S. Army Col. Ann Wright who is known for being one of three State Department officials to publicly resign in 2003 as a way to protest the U.S. invasion of Iraq.

See a CNN.com exclusive interview with Ellsberg as he talks about his kinship with Manning, and Ellsberg's experiences as an infamous war-time leaker.

Manning, a 23-year-old Army private, is accused of giving WikiLeaks hundreds of thousands of classified military and State Department documents. He is awaiting a decision on whether he will face a court martial.

The protesters marched near the base carrying placards, shouting slogans and chanting "Free Bradley Manning." They left flowers in front of an Iwo Jima monument at the entrance of the Quantico. Supporters of Manning and WikiLeaks also made speeches deploring Manning's alleged mistreatment in prison and asserted that the dissemination of secret military documents is vital for democracy.

Later police arrested dozens of protesters for apparently blocking an intersection leading to the base. Prince William County police did not provide CNN information about the arrested protesters Sunday night, including what possible charges they might face.

Reports that Manning has been held in near-total isolation and forced to sleep without clothing during his nearly eight-month detention has sparked outrage from human rights and peace activists. The controversy even played a hand in the resignation of former State Department spokesman P.J. Crowley, who reportedly suggested that the Defense Department is mistreating Manning.

Ellsberg told CNN that Crowley "acted honorably and boldly" for remarking to a small group at MIT that Manning's treatment had been "ridiculous, counterproductive and stupid." Crowley resigned days after his statement.

Ellsberg said Manning's imprisonment is illegal.

"The president could change this treatment of him. Apparently, (President Barack Obama) has been told by the Defense Department that this is appropriate," Ellsberg said. "That's a terrible commentary on our standards, which means that they feel free to use illegal measures against someone in their custody."

Obama told reporters a week ago that he had been assured by the State Department that Manning is being treated humanely.

Ellsberg said the Marines holding Manning should not wait for an order from Obama "to stop disobeying the law."

Ellsberg said his opinion is shared by legal experts across the country.

"As a Marine I am ashamed that the corps is doing this," he said.

Sunday evening, the protesters wrote their grievances in a open letter addressed to Obama and Defense Secretary Robert Gates.

In addition to Ellsberg and Wright, the letter is signed by a number of musicians and actors, including Rosanne Cash, Michael Stipe of R.E.M. and actors Danny Glover and Viggo Mortensen.

David House, a friend of Manning's, disputes that. House told CNN that he has visited Manning nine times since the soldier was taken into custody. House said that his friend recently appeared to be in "decline mentally and physically."

House said Manning also has been reluctant during recent visits to discuss his treatment. Manning's attorney David Coombs has said that his client has been put on suicide watch and forced to stand at attention while naked.

Military officials have said Manning has been forced to sleep naked to prevent him from committing suicide.

"There can be no conceivable justification for this type of degrading treatment," the letter from the activists says. "It brings back memories of the abuses committed in Abu Ghraib, which blackened the reputation of America's armed forces," said the letter, referring to an Iraqi prison that was the center of a prisoner abuse scandal.

The letter demands an immediate investigation into the conditions of Manning's detention.

On Sunday, many of the 400 supporters, peace activists and veterans tried to enter an administrative building to deliver a letter to Base Commander Daniel Choike, but were stopped by police.

It was the second straight day of rallies -- and arrests -- for many of the protesters, including those from antiwar groups Veterans For Peace, Iraq Veterans Against War and Code Pink, who attended a demonstration in front of the White House on Saturday to mark the eighth anniversary of the war in Iraq.

For Ellsberg, Sunday marked his second arrest for the weekend.

In 1971, Ellsberg who had worked as a top Pentagon official, leaked thousands of classified documents that revealed that top American officials had for years deceived the American public about the Vietnam War.

Since the Pentagon Papers, Ellsberg has become an outspoken antiwar activist.

CNN's Ashley Fantz contributed to this report.