Malaysia's prime minister has denied chartering planes to move tens of thousands of voters into marginal constituencies for Sunday's election.
His party, the United Malays National Organisation (UMNO), said the flights had been paid for by supporters and not by Prime Minister Najib Razak's office.
They were routine efforts to help people get back home to vote, it said.
The opposition allege a number of other abuses in the lead-up to the closest election since independence in 1957.
One researcher found a suspiciously large number of voters over the age of 100 in the eastern state of Sabah.
The official election commission says it has done everything it can to ensure this is the cleanest election in Malaysian history.
But with the two coalitions running neck and neck in the opinion polls, allegations of fraud could undermine public trust in the result, the BBC's Jonathan Head reports from Kuala Lumpur.
Mr Najib and opposition leader Anwar Ibrahim have been making a final pitch for votes.
'Oppressive and corrupt'
Emails and passenger manifests allegedly leaked by staff at Malaysia's national airline alerted the opposition to what it believes is a concerted effort by UMNO to shift voters into marginal constituencies.
The emails refer to requests for chartered aircraft from the office of the prime minister.
However UMNO official Tengku Adnan Tengku Mansor denied any involvement by the prime minister, saying the flights had been "organised and paid for by friends of Barisan Nasional", the ruling coalition which UMNO dominates.
Among other allegations of irregularities, the daughter of Anwar Ibrahim says there are nearly 5,000 extra voters registered in her Kuala Lumpur constituency whom she cannot trace.
In a nationally televised interview late on Friday, Mr Najib appealed to voters for a "strong mandate" to help him make good on promises of reform.
"Definitely, with a strong mandate, we can do much better in the next five years," he said.
On a campaign stop in the north on Friday evening, Mr Anwar told cheering supporters: "This election is an election of the people fighting oppressive and corrupt rulers."
Former Prime Minister Mahathir Mohamad has told our correspondent that this election will be closer than in the past but the idea of the opposition winning the poll is "far-fetched".
He said Mr Anwar was not a suitable candidate to be prime minister because of his "moral character" and raised the issue of sodomy allegations against him, in a country where homosexual acts are illegal.
When our correspondent put to him that Mr Anwar had been acquitted of the charges and was an innocent man, the former UMNO leader replied: "In the eyes of the law he is innocent but in the eyes of the people, he is not."
Asked about politics in Asia, Dr Mahathir said: "I still think Asia needs strong leaders.
"They like to be democratic but they don't really understand democracy."
Mr Anwar, who now leads an alliance of three opposition parties, Pakatan Rakyat, once belonged to UMNO himself but fell out with its leaders.