The Coalition for Unity and Democracy (CUD) and the United Ethiopian Democratic Forces (UEDF) said on Saturday that unless their activists were released and allowed to monitor Sunday's vote, the election would be meaningless.
"If the government doesn't release these observers by tomorrow, we are not going to accept the results of the polls," said Beyene Petros, vice-chairman of the UEDF, which said several hundred of its monitors had been detained.
"In the absence of our poll watchers in the election, we don't see why we should accept the outcome of the election," he said.
The CUD said that more than 1000 of its observers had been arrested in what it said was a continuation of malfeasance that the ruling Ethiopian People's Revolutionary Democratic Front had been committing for weeks.
"If the election is about to take place without our observers, we can't accept that," said deputy CUD leader Berhanu Nega.
Petros and Nega made the allegations at a hastily called news conference in Addis Ababa more than 12 hours before voting was to begin in Ethiopia's third election since the ousting of a Soviet-backed dictatorship in 1991.
They could not offer precise numbers as to how many of their observers had allegedly been detained, but said the arrests had occurred over the past three days.
Government officials were not immediately available to comment on the accusations, but the chairman of the National Electoral Board of Ethiopia, Kemal Bedri, said he was taken aback by the scale of the accusations.
"I'm caught be surprise," he said. "I don't think it's true, but we are investigating."
The opposition allegations surfaced shortly after a human rights watchdog complained its observers were being hindered from getting to polling stations.
The Ethiopian Human Rights Council said the deployment of 1644 of its observers was being prevented by the election board in violation of a court order.