Fifteen people have been arrested for registering children as young as three years old to vote in May's election, say Ethiopian polling officials.
The fraud was discovered in Hadiya district, south of Addis Ababa, where elections in 2000 had to be re-run because of similar abuses.
National Election Board chairman Kemal Bedri told reporters that registration would have to be carried out again.
There has traditionally been strong opposition to the ruling party there.
"Children aged three had been issued with registration papers," he said. "We found that some people's names were registered three or four times."
He blamed negligent election officials and opposition party figures, who he said were trying to discredit the election process.
He added that the children involved would not be punished.
Opposition United Ethiopia Democratic Forces officials denied any part in the scam
"We are the ones who brought the case to the attention of the national election board, with photographic evidence," UEDF Chairman Beyene Petros told AFP news agency
He said that just last week they complained of fraud in registering voters.
Some 25 million Ethiopians are able to vote in the 15 May parliamentary and regional assembly elections. MPs then choose a prime minister.
Some 35 political parties will contest the polls, although to date the elections have always been won by the ruling Ethiopian People's Revolutionary Democratic Front.