The nation's electric power network is vulnerable to terrorist attacks that could black out large sections of the country, the congressional Office of Technology Assessment reported yesterday.

The report said what energy experts have long known but have been reluctant to discuss for fear of encouraging attacks on the power grid by terrorists or vandals. Sen. John Glenn (D-Ohio), who released the report, said one chapter had been withheld "based on objections from the Department of Energy and other security agencies that it provided too clear an analysis of how to damage the power system."

"Major metropolitan areas and even multi-state regions could lose virtually all power" in simultaneous attacks on remote substations and unguarded transmission lines, the report said. In one small area of North Dakota, for example, not far from the Canadian border, transmission lines from six power plants that serve several midwestern cities march across the landscape in unguarded isolation, approachable from any direction.

Attacks on power pylons and substations are standard operations in guerrilla wars such as in El Salvador, but they have been rare in this country. Earlier this year, however, an attack on a transmission line in California cut off power to 92,000 households. Pacific Gas and Electric Co. later offered a reward for information about possible suspects. No arrests have been made, a company spokesman said yesterday.

Glenn, chairman of the Governmental Affairs Committee, said that "no one is advocating a Maginot Line" around electric generation and transmission facilities. But he said the report convinced him that not enough has been done to ensure safety of the power grid.

John J. Easton Jr., assistant secretary for international affairs and energy emergencies, said "government and industry perceive the present risk to be low" and that "an existing multi-state sabotage threat has not been identified."

He cautioned against spending more money on fences, security guards and standby facilities than the level of threat warrants. But he endorsed efforts by the utilities, some state governments and a federal interagency planning group to assess the threat and develop plans for coping with an electricity shutdown.

He said he agreed with the report's observation that "before additional security measures are taken, utilities and {state} utility commissions will have to agree" on how much should be spent and who should bear the cost.