MANILA, THE PHILIPPINES -- Security forces in the Philippines were placed on full alert Thursday amid the discovery of a plot to assassinate President Gloria Arroyo, officials said.

Announcement of the plot came a day before Arroyo's political opponents were to stage a major protest to call for her resignation over allegations of corruption linking the first family. Arroyo's security chief, Brigadier General Romeo Prestoza, said the plot was being hatched by "extremists Jemaah Islamiyah (JI) and the Abu Sayyaf
Group," referring to Muslim militants with reported links to Al-Qaeda.

"It is not just the president, there are other targets," he said, adding that Arroyo has been informed of the threat, forcing her to cancel a scheduled trip to the northern resort city of Baguio Friday."If they want to launch it, they can do it anytime," Prestoza told reporters.

He said elements composed of militants from the Abu Sayyaf and Jemaah Islamiyah were also planning to hit "high-value targets" around Manila.Both groups are known to operate in the southern island of Mindanao and
have been blamed for the Philippines' worst terrorist attacks in recent years.They are however known to field 'cells' responsible for bombings around Manila in the past as well.

Army spokesman Captain Carlo Ferrer earlier Thursday said they have separately received intelligence reports that elements from the communist New People's Army (NPA) rebel group may infiltrate the ranks of protesters Friday
and instigate violence.He said there were intelligence reports that the rally might be sabotaged by the rebels. 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 in kickbacks from a telecoms deal with a Chinese firm.The 329-million-dollar 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. Leaders of the influential Roman Catholic Church, business leaders and even lawyers' groups have expressed support for the protest planned in the Makati business district.

The presidential palace has called for calm amid the turmoil and challenged Arroyo's opponents to file charges in court. On Thursday, US envoy to Manila Kristie Kenney said any protest should be put in the "framework of the constitution and the rule of law". However, she said the "right to public protest to express views and opinions is fundamental to all of us".

"It's important for government and for the private sectors to see that," Kenney said.