Events will be held across the world, including the UK, to mark the 1,000th day spent in prison by the United States soldier arrested over the leaking of classified documents to whistleblowing website WikiLeaks.
Private Bradley Manning was arrested in May 2010 in Iraq on suspicion of having passed classified material to WikiLeaks, whose founder Julian Assange has sought refuge inside the Ecuadorian Embassy in London.
A court martial is set to be held in June at Ford Meade in Maryland, with supporters treating him as a hero, but opponents describing him as a traitor.
A series of protests and demonstrations will be held across the United States and Europe, including London, Yorkshire and Cardiff.
"There has never been a more important time to broadcast our message of support for exposing war crimes, international justice, and people's right to know what the government does in our name," said a spokesman for a Bradley Manning campaign group in the United States.
Mr Assange has been in the Ecuadorian Embassy since last June, after suddenly arriving there as part of his campaign to avoid extradition to Sweden, where he faces allegations of sex offences - which he denies. He fears being sent to the United States if he travels to Sweden.
New York civil rights attorney Chase Madar has written a book about Bradley Manning, raising a number of issues including the soldier's solitary confinement for 11 months after his arrest.
Mr Madar told the Press Association that he believed that Manning was a "tragic hero". He has written to the soldier, and sent him a copy of the book, but has never received any replies, possibly on the advice of Manning's lawyer.
The author said most people in the United States did not know who Bradley Manning was, and only a minority would regard him as a national hero.
"There is still a perception that the US is at risk from military leaks - but I believe what he did was a good thing, both for the United States and for the rest of the world. The documents leaked represented less than 1% of what Washington classified in the whole of 2011."