Troops have surrounded a seminary in Manila which is sheltering a former intelligence officer who claims to have evidence of presidential corruption.
The military denies it plans to storm the seminary
Samuel Ong says he has copies of tapes in which a woman he says is President Gloria Arroyo apparently conspires with an election commissioner.
Mrs Arroyo has said the tapes were doctored to falsely implicate her in fraudulently winning last year's poll.
The row has fanned political tensions caused by Mrs Arroyo's unpopularity.
Correspondents say there is widespread public discontent with her government, amid repeated allegations of official corruption and a poorly performing economy.
Mr Ong has been hiding in the seminary since Friday, when he admitted to the media that he was the source of what he called "the mother of all tapes".
In the recording, a woman he says is Mrs Arroyo allegedly asks an election official during the count for the 2004 election whether her lead could fall below one million votes. The official says her lead has fallen, but that some wards have still to be counted. The official then says "we will do our best".
Mr Ong, dismissed deputy head of the Justice Department's investigation agency, said he had been given the tape by intelligence officers disaffected with Mrs Arroyo's rule.
"I will be dead meat when I leave this place," Mr Ong said in a radio interview from the seminary on Monday.
"I will not be arrested alive," he said.
But Metro Manila police chief director Vidal Querol insisted the two armoured personnel carriers, trucks of soldiers and 150 anti-riot police in the area were not about to storm the seminary.
Samuel Ong went public about his role on Friday
The army is also on alert for more anti-government protests, after thousands massed in Manila over the weekend.
Opposition senators say Mrs Arroyo fixed last May's election and also accuse members of her family of taking pay-offs from illegal gambling syndicates. The accused deny all the allegations.
Mrs Arroyo beat popular film star Fernando Poe by 3% of the vote to win her first electoral mandate since she assumed the presidency in 2001. As vice-president, she had taken over the presidency after a popular revolt against her predecessor Joseph Estrada.
Mr Poe died of a stroke in December, while Mr Estrada remains in prison on corruption charges.
The Philippines has seen two popular revolts and at least a dozen coup attempts in 20 years.