MANILA, Philippines ? (UPDATE) Philippines security forces were on high alert Thursday amid concerns that communist rebels planned to infiltrate protests to demand the resignation of President Gloria Macapagal- Arroyo over mounting corruption allegations.

Troops were placed on "full red alert status" from Wednesday night to prevent any violence leading up to Friday's demonstration organized by Arroyo's political opponents and several leftist organizations.

Captain Carlo Ferrer, military spokesman, said intelligence reports indicated the communists planned to disrupt an opposition-led rally Friday "to create confusion and chaos."

"Organizers are advised to police their own ranks so that they will not be infiltrated and have an orderly and peaceful mass action," Ferrer said in a statement.

National police chief Avelino Razon Jr. said authorities were trying to verify the alleged communist plot to hijack the opposition rallies.

Police will guard against violence and ban protests at a popular pro-democracy shrine and near the presidential palace in Manila, Razon told The Associated Press. Security personnel were erecting checkpoints in key points across the capital.

The rebels vowed Wednesday to intensify attacks to weaken the government, but they have largely refrained from assaults that could hurt civilians during their four-decade insurgency.

Opposition leaders dismissed the military and police announcement as a ploy to discourage crowds from joining the protest in Makati, Manila's financial district.

Political tensions have increased since the dramatic emergence last week of corruption witness, former government consultant Rodolfo Lozada Jr., who linked a former elections chief and Arroyo's husband to an allegedly overpriced $329 million government broadband contract in a Senate testimony. Both men have denied the allegations.

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 Makati.

The Palace has called for calm amid the scandal, but has 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.

Arroyo has survived three opposition impeachment bids and four attempted power grabs, mainly due to the support of loyal generals and a formidable political coalition during her seven tumultuous years in power.

She has been accused of rigging the 2004 election and was later implicated by opposition politicians in a series of corruption scandals, along with members of her family. Arroyo has rejected the allegations and vowed to finish her term until 2010.

The government is afraid that the upcoming protests could snowball into a nonviolent "people power" revolt, similar to the uprisings that toppled dictator Ferdinand Marcos in 1986 and Joseph Estrada in 2001 over allegations of massive corruption and misrule.

Arroyo, who helped lead the 2001 protests, succeeded Estrada and won a six-year mandate in 2004.