Two protesters will trek 60 miles from Brighton to London to demand the release of Guantanamo Bay detainee Omar Deghayes.
Glenn Williams and Sally Griffiths, of the Save Omar campaign, will begin their marathon walk on Saturday from the Palace Pier, Brighton.
They hope to reach Whitehall three days later.
Saltdean resident Omar Deghayes has been held in Guantanamo Bay for more than four years without charge.
Mr Williams, of Park Crescent Road, Brighton, said: "I have never done such a long walk before and it will be a real challenge.
"But the campaign is increasingly frustrated at being ignored by the Government and we will try anything to get them to listen."
Save Omar campaigners will gather at the pier at 8.30am and will walk with the pair for the first few miles.
They will then greet them outside Brixton's underground station on Tuesday at 12noon to walk to 10 Downing Street where they will hand a protest letter to Prime Minister Tony Blair.
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