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 .
Thousands of demonstrators protest the treatment of Army private . 1971 Pentagon Papers lists Daniel Webster among the protesters . Ellsberg and former Army Col . Ann Wright are among dozens arrested .