Bulgarian authorities said they had seized 350,000 illegal ballot papers ahead of yesterday's elections, an allegation which sparked opposition accusations of fraud against the former ruling party of Boyko Borisov.
Socialist party leader Sergey Stanishev said the discovery was "preparation for total falsification of the elections".
"350,000 ballots correspond to 10 per cent of the expected turnout tomorrow and ensure about 25 lawmakers' seats," he said. "This is a scandal unseen in Bulgaria so far."
Stanishev accused the conservative Citizens for European Development of Bulgaria (Gerb) party of ex-premier Borisov and his campaign manager, former interior minister Tsvetan Tsvetanov.
The papers in Kostinbrod, 15 kilometres northwest of Sofia, exceeded the number that the 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 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.
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.
Interim technocrat Prime Minister Marin Raykov urged parties "to not use this case to break the silence of what should be a day of reflection".
Vote-buying and other concerns prompted the Organisation for Security and Co-operation in Europe to send its biggest monitoring team to Bulgaria since 1990 for Sunday's vote.
Five parties - but not Gerb - have also ordered an independent parallel vote count, prompting fears that Bulgaria might see its vote results challenged for the first time since 1990.