Three Venezuela opposition parties have pulled out of Sunday's congressional poll, accusing the electoral body of favouring pro-government candidates.
The opposition is demanding equal conditions in the election
The head of the main opposition party, Democratic Action, said they felt the result would be biased against them.
However, Vice-President Jose Vicente Rangel said the party was pulling out because it was facing defeat.
Venezuelans will vote for an expanded 167-seat congress, where supporters of President Hugo Chavez have a majority.
Correspondents say that the government has vowed to increase its majority to two-thirds, which would allow it to pass constitutional reforms that opposition leaders strongly oppose.
Democratic Action chief Henry Ramos said his party was demanding a suspension of the elections until equal conditions existed for parties.
He told a news conference the electoral board had not convinced opposition candidates that the software used in the computerized voting system did not endanger voters' confidentiality.
He said the decision to pull out had been a difficult one to take at such a late stage in the campaign.
"Imagine what it means to us to say today that under these conditions we cannot participate in the electoral process."
Project Venezuela and the Social Christian Party, or Copei, later said they too were withdrawing and called for a suspension.
Members of the electoral council have repeatedly denied accusations of a pro-government bias.
Vice-President Rangel defended Sunday's poll.
"It's the cleanest in Venezuela's history, but they have interests opposed to the National Electoral Council," he said.
Earlier, a close ally of President Chavez accused Washington of interfering in the congressional elections.
National Assembly Speaker Nicolas Maduro said the US embassy was working with opposition groupings such as Sumate to encourage abstention and scepticism about the neutrality of the election board.
The poll on Sunday will be overseen by observers from the Organization of American States and the European Union.