Security forces in the Philippines have been placed on full alert.
"It is not just the president, there are other targets," he said. "If they want to launch it, they can do it anytime."
He said the plan did not appear to be connected to the opposition rally at the Makati business district in Manila, planned for Friday.
Army on alert
General Hermogenes Esperon, the Philippines armed forces chief, said news of the plan "had become the basis of our action for putting the armed forces of the Philippines in full state of preparedness".
He said elements comprising Abu Sayyaf and Jemaah Islamiyah fighters were also planning to hit "high-value targets" around Manila.
Both groups, which have been blamed for the worst attacks in the Philippines in recent years, are known to operate on the southern island of Mindanao.
They are, however, known to field "cells" responsible for bombings around Manila in the past as well.
Earlier on Thursday, Captain Carlo Ferrer, the army spokesman, cited intelligence reports that elements from the New People's Army (NPA) communist group may infiltrate the ranks of protesters on Friday and instigate violence.
The NPA is the armed wing of the Communist Party of the Philippines, which has been waging a Maoist rebellion since 1969.
Arroyo's critics have been holding daily protests around Manila calling on her to resign over fresh claims that her husband and a political ally tried to get millions of dollars worth of bribes from a telecoms deal with a Chinese firm.
The $329 million deal for a national broadband network with China's state-run ZTE has since been cancelled by Arroyo.
Some business groups have warned that the scandal could plunge the country into a new round of political instability and dampen investor confidence.