Colombia's armed forces launched a huge operation Saturday to stop leftist guerrillas from disrupting Sunday's national elections to a constituent assembly after rebels ambushed and killed four soldiers.

About 230,000 members of the police, army, air force and navy are involved in the "Democracy Plan" to protect polling stations nationwide, a Defense Ministry spokesman said.Voters will elect 70 members of an assembly which will meet for five months next year to reform Colombia's constitution. Many Colombians see reform as the key to broadening democracy and reducing violence.

Some 100,000 troops have reinforced remote regions and 25,000 more are being held in reserve as a rapid reaction force in case of guerrilla attacks, the spokesman said. This force includes a paratroop company which can be flown quickly to trouble spots.

The official said a rebel ambush on an army patrol Friday night in the southern province of Caqueta was a sign that the guerrillas wanted to disrupt Sunday's polls.

Newspapers said about 30 rebels of the Marxist Colombian Revolutionary Armed Forces (FARC) ambushed the patrol near Florencia, 190 miles south of Bogota.

Four soldiers were killed and five wounded in the gun and grenade attack, the reports said. Several guerrillas were also reported killed.

The Defense Ministry spokesman confirmed four soldiers had died but had no others details.

Several Colombian guerrilla groups have either disbanded or are holding peace talks with the government. But the two biggest and most radical, the FARC and the National Liberation Army, fight on. They traditionally seek to disrupt elections with attacks or by stealing ballot boxes in remote areas.