As many as 77 people were feared dead today in heavy fighting that erupted in mountains on the edge of Bogota as hundreds of Marxist rebels prepared to attack the capital, the Colombian army said.

An army spokeswoman said 40 crack counterinsurgency troops and 37 Revolutionary Armed Forces of Colombia guerrillas died in the battle that began at dawn around the Andean village of Gutierrez, 15 miles south of Bogota.

She added that two other army units, totaling 96 men, were missing after they lost radio contact with their company.

The fighting was the worst in a series of rebel attacks and appeared to be the heaviest defeat for the military since the government and the rebel group--known by the Spanish initials FARC--launched formal peace talks in early January.

The negotiations, which are going ahead without any prior cease-fire agreement, are aimed at ending Colombia's long-running civil war that has killed more than 35,000 people in 10 years.

"There are a large number of dead on both sides. I don't know what the rebels' targets were but they were coming toward Bogota and were going to carry out attacks in Bogota or the outskirts," said armed forces chief Gen. Fernando Tapias.

"This is an act of war."

Tapias and other army officials met with President Andres Pastrana early tonight.

An army statement said one of the FARC's most "bloodthirsty terrorists," known by his nom de guerre "Romana," was leading the 500-strong guerrilla column.

At least 1,000 soldiers had been sent into the area of towering peaks and deep chasms, situated almost 10,000 feet above sea level, by dusk today as fighting raged.

Reinforcements from the Bogota based 13th Brigade, the No. 1 Mobile Brigade and U.S.-trained Special Forces were being flown into the combat zone.

Army Col. Paulino Coronado said he believed Romana had been planning a high-profile show of force in Bogota to strengthen the rebels' position at the peace talks.