The Roman Catholic Church in the Democratic Republic of Congo has called for a boycott of Sunday's elections if alleged vote-rigging is not addressed.
Catholics make up more than half of DR Congo's 56m people
Confusion over the number of registered voters could lead to attempts to rig Sunday's polls, according to a letter read out in churches in Kinshasa.
Catholic bishops said on Friday they would not recognise the poll results unless their concerns were addressed.
More than half of the civil war-torn country's population are Catholic.
The United Nations' special envoy, William Swing, has said preparations for the elections are going very well, although much still needs to be done.
Mr Swing told the BBC that the UN was vigilant but not overly anxious about the security situation, and he welcomed the fact that militias in the east had not disrupted the election process.
DR Congo is holding its first-ever free presidential election after a damaging civil war which officially ended in 2003.
However, more than 1,000 people a day are still dying as a direct or indirect consequence of the conflict.
Warlords as candidates
UN Secretary General Kofi Annan earlier called the election a "logistical nightmare" and the election faces problems before it has even been held, BBC World Affairs Correspondent Mark Doyle reports from the capital, Kinshasa.
Catholic leaders are among powerful groups calling for a boycott of the polls.
They argue that the incumbent President, Joesph Kabila, has mobilised state resources to back his campaign and is using intimidation as a political tool.
Another fundamental issue is that the candidates with biggest campaigns are all former warlords who caused many of the country's problems in the first place, our correspondent says.
They fought their way to the negotiating table and then obtained top positions in the post-war transitional government, which ends its mandate with next weekend's polls.
Mr Swing believes that a successful free election and the stability that could bring would benefit not just DR Congo but the whole of Africa.
"This is arguably the only sub-region in Africa that has always lacked any centre of political stability and because of the size of this country, with nine neighbours, it is the only country that can give it that stability," he said.
"If the crisis in the Congo can be successfully resolved, Congo can change the face of Africa. Very few other crises can."