Phase one of Boston City Hospital's extensive service cutbacks will go into effect today, as the institution that has catered to the health needs of Boston's poor for a century apparently moves full speed ahead with its master plan to reduce the number of its beds from 830 to 500.

The trustees of Boston's Department of Health and Hospitals announced Tuesday that it would slash by one-third the hospital's pediatric services after seven private hospitals agreed to absorb the patient overflow.

Ultimately, and perhaps by July 1, the hospital's bed capacity will be reduced to 70 in obstetrics and gynecology, 175 in medicine and 200 in surgery, in addition to the 55 in pediatrics that remain following today's action.

The first of the cutbacks comes a month after the trustees gave jurisdiction over the hospital's services to Boston University. That decision was the culmination of a battle between Harvard, Tufts and B.U. to gain sole administrative responsibilities.

The controvery arose following the drastic cutbacks in the hospital's budget by Boston's City Council.

The settlement reached with the private facilities is a landmark agreement among Boston's hospitals.

It provides that a hospital admit any child to an available bed on the recommendation of a doctor from another hospital or neighborhood health center.

It assures that if the hospital is full, the alternative hospital for admission will be the preference of the patient or his family.

Furthermore, if the patient requires specialized care available at only one hospital, he will be admitted there.

Francis E. Guiney, director of BCH, said Tuesday that pediatrics was one of the easiest places to begin bed capacity reduction since only about 70 per cent of the inpatient facilities of that service are presently utilized.

As a result of the March decision of the trustees to award control to B.U., the composition of BCH's internship program will change drastically.

While in years past, the hospital's 30 internship positions were divided equally among three schools, only one individual from an applicant pool of 500 will be appointed to the Harvard services this year. The number of B.U. interns at the hospital will increase by nine, these previously having been Harvard appointees.

Thirty per cent of the clinical training of Harvard students has been done at BCH. Dr. Franklin H. Epstein, professor of Medicine, said yesterday that Harvard will conduct the same teaching of the same number of its own students with Harvard faculty at BCH next year. He estimated that the school will continue to maintain its current level of research and teaching at the hospital for three to five years.

However, he suggested that it "would be only sensible" for Harvard to begin to look elsewhere for additional facilities, since, he said, modern teaching of medicine implies control of the teaching hospital's house staff