Philippine President Gloria Macapagal Arroyo has admitted phoning an election official during last year's poll.
Mrs Arroyo said she was sorry, but added she would not resign
In a televised speech on Monday, she apologised for her "lapse of judgement" but denied trying to influence the vote and insisted she would not resign.
Critics said a tape recording of a phone call between a woman sounding like her and an election commissioner was proof she tried to rig the vote.
The political crisis comes as her support has hit an all-time low.
In her national address, Mrs Arroyo did not directly refer to the tape recording, which was released to the media three weeks ago.
But she did say she had spoken to an election commissioner about her share of the vote in the May 2004 poll.
"I was anxious to protect my votes and during that time had conversations with many people, including a Comelec (commission on elections) official," she said.
"My intent was not to influence the outcome of the election and it did not."
The president rejected calls to step down, saying the "outcome had been predicted by every major public opinion poll" and judged to be fair by international observers.
Telephoning an election official during the vote is not illegal in the Philippines, but the opposition is still likely to see it as a clear breach of conduct.
Critics have been calling for Mrs Arroyo's resignation ever since the tape recording surfaced, citing the phone conversation as evidence she tried to rig the election.
Mrs Arroyo's critics have held protests calling for her resignation
The tape itself has received very close media scrutiny, and part of it has become a hugely popular ring tone on mobile phones.
As the furore rumbled on, a spokesman for Mrs Arroyo suggested the tapes had been doctored but the president herself chose to say nothing until Monday's statement.
In the recording, a woman sounding like the president allegedly asks an election official whether her lead could fall below one million votes.
The official replies that her lead has fallen, but that some wards have still to be counted. The official then says: "We will do our best".
In the event, Mrs Arroyo beat the now-deceased film star Fernando Poe by more than a million votes.
In her address on Monday, the president said she wanted "to close this chapter and move on with the business of governing."
"I want to assure you that I have redoubled my efforts to serve the nation and earn your trust," she told Filipinos.
But that trust may be difficult to regain easily.
In addition to the tape recording controversy, the president also faces separate allegations that members of her family took pay-offs from illegal gambling syndicates.
She is also losing popularity due to the poorly performing Philippine economy, and proposed budget reforms.