Bulgarian authorities said Saturday they seized 350,000 illegal ballot papers a day before elections, sparking furious opposition accusations of fraud against the former ruling party of ex-premier Boyko Borisov.
Socialist party leader Sergey Stanishev said the discovery was "preparation for total falsification of the elections."
"350,000 ballots correspond to 10 percent of the expected turnout tomorrow and ensure about 25 lawmakers' seats," he said.
"This is a scandal unseen in Bulgaria so far," Stanishev fumed, directly accusing the conservative GERB party of Borisov and the ex-premier's campaign manager, former interior minister Tsvetan Tsvetanov.
The papers in Kostinbrod, 15 kilometres (10 miles) northwest of Sofia, exceeded the number that the privately owned printing company Multiprint was under contract to print.
"The ballots found on the night of May 10 were outside the ordered amounts," prosecutors said in a statement.
The government confirmed that it had indeed received the whole order of 8,343,000 ballots from Multiprint on May 8.
The owner of Multiprint denied any wrongdoing, saying that the extra papers were due to be thrown away. Prosecutors said however that they were packed up and ready to be despatched.
The operation overnight was carried out by agents of Bulgaria's State Agency for National Security (DANS) and prosecutors, the statement said.
Bulgaria's state BNR radio said that Multiprint's owner was close to one of Bulgaria's key political parties, and that the interior ministry was deliberately excluded from the operation.
Private BGNES news agency named the party as Borisov's GERB, whose deputy leader Tsvetanov headed the interior ministry until GERB's ousting from power in end-February.
The prosecution did not comment on this information.
The allegations come at an awkward time for Tsvetanov, already embroiled in late April in a scandal about alleged illegal wiretapping of the party's opponents and businesspeople.
The opposition reactions came despite a call from interim technocrat Prime Minister Marin Raykov to the campaigning ban on the eve of polling day.
He urged parties "to not use this case to break the silence of what should be a day of reflection."
Lyutvi Mestan, the leader of the Turkish minority MRF party expected to come third in the election, said: "This is a coup attempt, for a forceful usurpation of power."
He insisted on having the signatures on all officials in polling stations on each cast ballot to guarantee that no papers were brought from outside.
The right-wing DSB party of former premier from 1997 to 2001 Ivan Kostov pressed for a postponement of the vote until new ballots were printed out.
GERB had denied the accusations as a campaign aiming to smear them and filed a complaint against the television channel that broke out the news.
GERB is forecast to come first in the election, three months after mass protests about poverty forced Borisov's government to resign, but to fall well short of a majority.
Vote-buying and other election fraud concerns have prompted the Organisation for Security and Cooperation in Europe (OSCE) to dispatch its biggest monitoring mission to Bulgaria since 1990 for Sunday's vote.
Five parties -- but not GERB -- have also ordered an independent parallel vote count, prompting analysts to fear that Bulgaria might see its vote results challenged for the first time since 1990.