The cordon is being set up by the French authorities because of fears of a terrorist attack or a repetition of clashes between demonstrators and police, which have marked previous G8 meetings.
Campaigners hope to lobby G8 leaders to do more to cancel debt in Africa, billed as one of the main themes of the summit this year.
"It's not clear what access there will be, and the press centre is eight miles from where the summit meeting will happen," said George Gelber, head of public policy at Cafod, which is sending two staff to the meeting.
"It will be an uphill struggle because at G8 there is a heavy security cordon and because contact with NGOs is always really stage-managed."
The Jubilee Debt Campaign said it was sending a handful of staff and volunteers to Evian, on the southern shore of Lake Geneva, but is not encouraging supporters to go because of the security cordon. Hundreds of British campaigners went to Genoa in Italy to lobby the G8 in 2001, when one man died in clashes with police.
The campaign has instead called on supporters to attend an alternative summit in Birmingham called the Day to Remember conference in the run-up to the Evian meeting.
The conference on 16 May will mark five years since 70,000 people formed a human chain around the G8 meeting when it took place in Birmingham in 1998.
"We're five years on, and we've still had such a limited programme on cancelling debt. There's so much more the G8 needs to do," said Ashok Sinha, co-ordinator of the Jubilee Debt Campaign.
The conference will feature speakers from the developing world as well as a debate with local MPs and religious services. Organisers are also hoping to stage another human chain in the city centre.
Campaign groups including Tearfund, Christian Aid and Cafod hope supporters will send nearly 250,000 anti-debt postcards to French President Jacques Chirac, who is hosting the G8 summit. More than 200,000 postcards have been printed with a one euro coin pictured on the front.
"We're making the point that more than 300 million Africans are living on less than one euro a day - that's 65 pence," said Sinha.
President Chirac has put debt to Africa and access to clean water for the world's most impoverished countries among the major themes for this year's summit.
But campaigners are now worried that the fallout from the conflict in Iraq will knock these issues lower down the agenda.
"There's a danger that George Bush will refuse to even be in the same room as the French president and, if so, we've really got a problem," said Gelber.