Mayor Walter E. Washington called area utility executives and city department heads together yesterday for reassurance that the District of Columbia would not collapse in the event of a major power blackout. The mayor got what he wanted.

"We have a plan," he told reporters after a closed discussion and briefing from executives of the Potomac Electric Power Co. (Pepco). "I am assured that although nature still might wreak its havoc, we are prepared to deal with the problem."

The department heads, he said, had reviewed the plan put together two years ago by the D.C. Office of Emergency Preparedness and found it needed no revision. "We tested it around the table here," he said.

The 28-page plan, dated July 3, 1975, contains few specific actions the city government would take in case of what the report calls "a power outage emergency," the most extreme of three "situations" examined in the plan.

"It is the general responsibility of each department and agency to mobilize their manpower and other resources to assist in the areas of health, welfare, information and emergency preparedness," the plan says. "If a power outage (which may not be anticipated) occurs citywide, or affects large areas of the city, the department heads will be informed by the mayor's command center."

The plan then lists "emergency actions" that include instructions that department heads contact the command center, that they ensure the safety of occupants of their buildings, that the personnel office will decide on early dismissal of nonessential employees, and that the General Services Department will take "appropriate action" for multi-use building safety.

Responsible department heads, the plan continues, "will verify operations of any standby generating equipment," while the mayor's command center "will aid in providing emergency communications."

Asked later why the provisions were so vague, plan author Richard Bottorff of the Office of Emergency Preparedness said the plan was not intended to be more than "a guideline to the agencies for the development of their own plans."

The agencies were asked for copies of their plans at first, he said, "but we've since come off that . . . one place just can't maintain it. If you have 14 to 15 agencies you've really got something on your hands. The agencies themselves have the responsibility of keeping their plans current."

Inquiries by The Washington Post revealed that most of the city's agencies do have plans to deal with a power shortage or blackout emergency.

Police Chief Maurice J. Cullinane, asked for details on his plans to deal with possible looting like that that occurred during New York's blackout Wednesday night, said he was prepared to integrate his operations with those of the Fire Department and the National Guard, through the mayor's emergency command center.

"We feel very comfortable that we could stop any major looting," he said, "though of course in isolated areas where areas go dark, you're going to have some of that."

Pepco president and board chairman W. Reid Thompson said the discussion had been "most helpful in understanding the interplay between the staff and the agencies," and commended Mayor Washington for having had the plan prepared to advance.

He repeated his organization's position that the possibility of a New York-style blackout here is "very remote . . . but then you're always vulnerable to acts of God."