The D.C. Fire Department abruptly canceled a news conference yesterday at which Fire Chief Theodore Coleman was to respond to a highly critical report on the city's ambulance service.

Fire department spokesman Peter Woolfolk and Deputy Chief Ray Alfred told about 20 reporters and photographers gathered at department headquarters that they did not know who was responsible for canceling the scheduled 3 p.m. news conference.

At the same time, Woolfolk released four letters praising the ambulance service, a division of the fire department. The letters were written this year by a local doctor, the public affairs office at Children's Hospital, a nun and "a Rockville resident" who was not identified.

Coleman was scheduled to release a 30-page report responding to a study by a task force of Mayor Marion Barry's Emergency Medical Services Advisory Committee, which found that city's ambulance service keeps poor medical records, uses substandard equipment, has virtually no quality control mechanisms and is staffed by poorly trained and poorly supervised workers.

Woolfolk said he understood that the news conference was canceled because, along with Coleman's report, officials want to release a letter from the "medical community" praising the city's ambulance service, and the letter had not arrived.

Woolfolk and Alfred said they thought it was important that the department make a "timely" response to the report, which was given to the mayor last Thursday, but that they did not know when the news conference would be rescheduled.

"I think that we have adequate, clear . . . rebuttals to the accusations made in the report," Woolfolk said yesterday. He said the department will respond "item by item" to the 82 cited deficiencies and more than 140 recommendations in the report, but that it has taken time to collect the necessary data and documentation.

Local doctors have said that the problems with the ambulance division described in the report have existed for years, but that the city has not moved fast enough to correct them.

Some ambulance service workers also have agreed with the report. One paramedic, who did not want to be quoted by name, said there are chronic shortages of equipment and medications and that requests for supplies go unanswered for months. Other ambulance workers said they sometimes must buy their own materials or pilfer them from local hospitals.

In an April 23, 1984, memo to Coleman, Deputy Chief William L. Mullikin, then head of the ambulance division, complained of "critical deficiencies" in supplies that extended for months because of inadequate funding.

The task force report also criticized the ambulance division command structure and the lack of firefighters with medical training.

Tom Tippett, president of the International Association of Fire Fighters, Local 36, which represents the city's firefighters, said recently, "If I were injured today, I'd prefer that a fire truck take me to the hospital, and if that weren't available, I'd rather have a cab" than be transported by a city ambulance.