At the Election Court - part of the High Court - in London, four voters alleged mayor of Tower Hamlets Lutfur Rahman had committed electoral fraud.
Independent Mr Rahman was elected for a second term in May 2014. His lawyer said the claims were a "mixture of unfounded and false allegations".
The group of voters wants the poll declared void and rerun.
Evidence is to be outlined to Election Commissioner Richard Mawrey - a senior lawyer sitting as a judge - over a number of weeks.
The group of voters mounted the challenge under the provisions of the 1983 Representation Of The People Act.
Andy Erlam, who stood as a councillor on an anti-corruption ticket and is heading the group, said there were "serious questions" which "need answers".
Francis Hoar, representing the group of four, told Mr Mawrey his clients were accusing Mr Rahman of election fraud.
He said there had been "personation" - where people pretend to be someone else to cast a vote - in postal voting and at polling stations.
He added people had registered themselves or others to vote at addresses at which they did not live and there had been tampering with ballot papers.
Mr Hoar also alleged Mr Rahman had used "undue influence through intimidation at polling stations" and "interference with voters" - including in polling booths.
The court heard one presiding officer reported there was a "constant battle" to stop men entering polling booths with women and telling them how to vote.
He said there had been "bribery" through "unlawfully diverting public funds to organisations in order corruptly to procure their political support".
Mr Hoar told Mr Mawrey that his clients aimed to call almost 100 witnesses who "know about corruption" and "know about Lutfur Rahman".
Mr Mawrey told the court that, if he found allegations had been proved, he could declare the election "void" and rule that Mr Rahman had not been declared mayor.
Allegations against returning officer John Williams have been withdrawn by lawyers for the group, the court was told.
Timothy Straker QC, for Mr Williams, said it had been accepted his client had "acted properly throughout the election".
Mr Williams said, outside court, in a statement: "This was a hotly contested election run in challenging circumstances.
"As returning officer I am politically neutral and my only concern is to run an efficient, free and fair election."
Duncan Penny QC, for Mr Rahman, described the petition as "a rich and elaborate mixture of unfounded and false allegations" and its one aim was to unseat a directly elected mayor.
He told the court: "There is little, if any evidence, of personal wrongdoing by Lutfur Rahman" and described the group's claims as "invention" and "exaggeration".
He said Mr Rahman did not want to be elected as a result of fraudulent activity and had been anxious that electoral law was observed.
Mr Penny said: "He has consistently called for anyone with suspicion of malpractice to report them and has been a public proponent of reforms to the postal voting system."
The hearing continues.
The mayor of an east London borough has been accused of "corrupt and illegal practices", at a special court hearing.