BOGOTA, COLOMBIA, FEB. 8 -- At least 63 people have been killed, including 21 soldiers and policemen, in a major offensive by leftist guerrillas opposed to government-sponsored deliberations over a new Colombian constitution, authorities said today.

The attacks, the largest offensive in memory by Marxist guerrillas, left 32 guerrillas and 10 civilians dead in three days of heavy fighting, authorities said. A key oil pipeline was reported severely damaged.

Guerrillas of the Revolutionary Armed Forces of Colombia (FARC) and National Liberation Army (ELN) launched the assaults to protest the installation Tuesday of a 70-member assembly that has begun rewriting the constitution to modernize the government and broaden political participation.

The guerrillas, who claim to represent a broad spectrum, have denounced the assembly as illegitimate, contending that it is not representative of Colombian society, and they have sought to take part in the deliberations without giving up their arms, a demand the government has rejected. Government officials describe the attacks as aimed at forcing the assembly to admit representatives of the armed rebel groups.

The clashes have caused widespread alarm because the rebels have traditionally operated in isolated rural areas. "There is cause for concern but not for panic," Interior Minister Humbero de la Calle said Thursday night in announcing a plan to increase military patrols to combat the attacks. "There is no danger to the state, and our armed forces have the capacity to respond and protect its citizens."

While international attention has focused on Colombia's war against cocaine barons, ELN and FARC -- the oldest and among the most orthodox Marxist guerrilla movements in Latin America -- began the offensive under the banner of a coalition calling itself the Simon Bolivar Guerrilla Coordinator. ELN and FARC have sought to overthrow the state for more than three decades.

In this week's attacks, guerrillas fought troops less than 10 miles outside Bogota, the capital. They occupied small towns, set up roadblocks on main highways across the country and burned 73 buses to disrupt public transportation around Bogota, Medellin and other large cities.

A guerrilla roadblack Thursday night on a highway on the northern coast was reported by Colombian journalists to have delayed about 1,000 vehicles and 5,000 travelers for 12 hours. No clashes or injuries were reported.

The rebels blew up three sections of the Cano Limon-Covenas oil pipeline, spilling thousands of barrels of crude oil into the jungle and rivers on the northeast coast. The guerrillas targeted electrical and communications facilities, cutting off parts of the country. Authorities said the assaults caused millions of dollars in damage.

President Cesar Gaviria warned Thursday that the attacks could slow economic growth and spur inflation. Gaviria is facing criticism for his handling of drug-related violence, and the guerrilla attacks caused fear that right-wing paramilitary armies might respond by stepping up activities.

This morning, the right-wing appeared to strike back. Three leftist members of the city council in Yondo, 125 miles north of the capital, were taken from their vehicle and shot in the head by masked men. In an effort to stem the violence, de la Calle offered Thursday for the first time to open direct, high-level talks with the guerrillas outside the country without preconditions. Previously the government has demanded a cease-fire before talks could begin. Alvaro Leiva, a constitutional assembly member with ties to FARC, said today that he had talked to guerrilla representatives and that they would respond "positively."

The proposal for negotiations with the rebels is expected to increase pressure on Gaviria to hold direct talks with drug traffickers, officials said, but they added that the government is unlikely to agree. The traffickers have tried to portray themselves as deserving treatment similar to that shown the guerrillas.

FARC, with an estimated 6,000 armed guerrillas, and ELN, with about 4,500 adherents, were offered assembly seats if they demobilized, but both groups refused. A third group, the Popular Liberation Army (EPL), signed a preliminary agreement and will be given two seats if it demobilizes by March 1.