A coalition of Guinean opposition parties has said it will not take part in December's presidential election.
Lansana Conte has been unwell
Opposition leader Mamadou Ba said the government was planning to rig the poll in favour of President Lansana Conte and warned of civil unrest.
The European Union has announced that it will not finance or send election observers because of doubts over the conduct of the poll.
Mr Conte seized power in a 1984 coup and has since won two contested polls.
In 2001, Mr Conte, who suffers from ill health, won a referendum to end the age limits for presidential candidates and extend the term of office from five to seven years.
"They have already prepared documents to show the president won with 85%," Mr Ba told the BBC Focus on Africa programme.
He also warned that Guinea could descend into civil war like some of its neighbours if free and fair elections were not held.
"It will end up like Sierra Leone, Liberia or Ivory Coast because if we cannot discuss with the man in power, we will fight him the way he wants," said the leader of the Revival Front for Democratic Change (Frad).
He said he was not threatening armed insurrection but street protests, which could escalate.
But Territorial Administration Minister Moussa Solano said "I see no reason to boycott the vote" because "all conditions had been met so the election can be organised".
Announcing the decision not to back the poll EU special representative Hans Dhalgren pointed to the lack of an independent electoral commission and the lack of access to state media for opposition parties.
There have been pro-democracy protests in recent years
"We have not found these elections to meet the basic criteria for us to come in as election observers," he told Focus on Africa.
At the weekend, President Conte warned opponents not to try to seize power by force.
The former leader of a coup also told the army to steer clear of anyone urging it to stage a coup ahead of the vote.
"I will never accept that someone comes to power by force. Those who want power will have to wait until the elections and contest it under the banner of a political party," he said.