Coup rumors have swirled for more than a month amid public outrage over gasoline price increases and a Senate investigation into allegations that many local officials, along with Arroyo's husband and son, have pocketed huge bribes from illegal gambling operators.
Arroyo has denied the claims, saying the opposition fabricated them to undermine her rule.
Gen. Efren Abu, the military's chief of staff, said unidentified groups were trying to weaken Arroyo's rule and to agitate troops and the public into withdrawing support from her administration.
"The Armed Forces of the Philippines is concerned over intelligence reports that there are continuing efforts among certain sectors to destabilize our democratic system using all means fair and foul," Abu said in a statement, without elaborating.
Abu said the military was loyal to Arroyo and warned: "We shall resist without hesitation all clandestine efforts to undermine the unity within the (military) and break the chain of command or any seditious act that will threaten the existence of the republic."
Military spokesman Lt. Col. Buenaventura Pascual said there was no imminent security threat against Arroyo's administration or any concrete sign of unrest among troops. He added that military commanders have been told not to believe anti-government groups trying to enlist support to oust Arroyo's government.
The coup rumors intensified Sunday, when presidential spokesman Ignacio Bunye accused Arroyo's political opponents of planning publicize a fake taped conversation between Arroyo and an elections official in which the two purportedly discussed ways of rigging an election.
An opposition leader, Rep. Francis Escudero, said he was not aware of the existence of such a tape. He urged the government to focus its attention on poverty and other problems instead of wasting time on baseless allegations and fear.
Rumors of a possible coup spread through the capital more than a month ago after retired Gen. Fortunato Abat, who served as defense secretary under former President Fidel Ramos, called for a civilian-military junta to replace Arroyo and Congress because of "a crisis in leadership."
Ramos immediately distanced himself from Abat, who assured the public that any proposed changes in the leadership would be done through peaceful means.
The 117,000-strong military played key roles in the ouster of dictator Ferdinand Marcos in 1986 and ex-President Joseph Estrada in January 2001.
In July 2003, about 300 soldiers, including US-trained special forces troops, occupied a hotel and a nearby shopping mall in Manila's financial district to protest alleged corruption in Arroyo's government and the military.
The uprising was immediately put down.