City officials and emergency room personnel gave sharply differing testimony on the state of the District's emergency ambulance service during a congressional oversight hearing yesterday.

City Administrator Thomas M. Downs vigorously defended the service, saying that the average time an ambulance takes to get from a firehouse to a person's home now is 6.8 minutes, down from 10.5 minutes in April.

He cited other improvements, including changes in leadership within the city fire department and daily review of ambulance runs by public health officials. Downs said a nationally known medical professional will be appointed to head the service "within weeks."

But doctors and nurses from some of the city's busiest emergency rooms said that despite rhetoric from Mayor Marion Barry and other city officials, there have been few real improvements in emergency care.

They said that the quality of emergency health care in the city is unlikely to improve until medical professionals, rather than firefighters, manage the ambulance service.

"The bottom line here is we're talking about people's lives, whether somebody is going to get to the hospital on time, whether someone's going to get on the scene in time to save someone's baby or mother or father," said Margaret Moreau, educational coordinator for trauma sevices at the Washington Hospital Center and a member of various city panels on the ambulance service.

"I've almost reached the point where I don't think we can make a difference anymore," Moreau added, echoing frustrations expressed in testimony yesterday by various medical professionals.

From September to April, there were eight reported incidents of ambulances that were slow in getting to people who later died. City officials said they found no link between the delays and the deaths, but in several of the cases, ambulance dispatchers or emergency medical technicians were disciplined.

Downs said that 1,000 of the city's 1,270 firefighters have been trained in cardiopulmonary resuscitation and that each firefighter will receive training as emergency medical technicians.

In addition, Downs said, fire engines are dispatched immediately in emergencies so that even in cases in which ambulances are delayed, "an EMT-trained firefighter {will be} on the scene within two to three minutes to begin the critical stabilization of patients."

He also said that the fire department's training operation, which has been criticized as inadequate, is being revamped.

In July 1986, before the recent series of delays and deaths that have focused attention on the service, the Mayor's Emergency Medical Services Task Group issued a sharply critical report that was called inaccurate by Fire Chief Theodore R. Coleman and other city officials. The report cited inadequate equipment, poor communication, poor training, lack of medical control and organizational problems.

Dr. Harry Chen, assistant professor of emergency medicine at George Washington University Hospital and the task group's chairman, said yesterday that city officials recently had drawn up a plan based on the report's recommendations. But Chen said the report's authors "are concerned that it's too little, too late."

"This is a crisis time for emergency medical service" in the District, Chen said.