A box of postal ballots which allegedly "appeared from nowhere" at an election count in Birmingham were included in the poll, a court has heard.
The city council's chief executive Lin Homer said the 1,700 votes were declared valid after a heated debate.
She told the High Court inquiry how officials struggled to contain the row, which broke out at the National Indoor Arena (NIA) last summer.
Three Labour councillors deny collecting postal votes fraudulently.
Ms Homer told the hearing at the Birmingham and Midlands Institute that claims were made about the apparent emergence at the NIA of a ballot box filled with 1,700 postal vote forms.
Ms Homer, who is Birmingham's returning officer, told the court: "Assertions were being made that a box or boxes had appeared from nowhere and that the box or boxes were full of postal votes.
"The individuals protesting were asking for these votes to be counted separately.
"My recollection is that I had a clear assurance from the deputy returning officer that the box had always been at the NIA and it was just a question of when it was opened."
The court is investigating allegations made by members of the People's Justice Party (PJP) that the successful Labour candidates and their agents in the Bordesley Green ward were involved in "grossly improper" fraud in the 10 June election.
Labour councillors Shah Jahan, Shafaq Ahmed and Ayaz Khan, deny any wrongdoing.
They walked out of the court proceedings last week after an application for the hearing to be adjourned was rejected.
The petitioners claim correction fluid was used to change votes marked for the PJP, the Conservative Party and the Liberal Democrats into votes for the Labour Party.
It is claimed that the fresh crosses were made in the same handwriting and with the same blue pen.
Ms Homer said she found it difficult to believe that anyone could have got through stringent security at the NIA with a ballot box that was unaccounted for.
The court also heard evidence from handwriting expert Michael Allen.
He was given 213 electoral documents to analyse as a sample of ballots cast in favour of Labour that were believed to be suspicious.
His findings suggested at least 80% of those ballots had been faked.
The hearing was adjourned until Tuesday.