The Carter administration told Congress yesterday that should the resumption of the military draft become necessary it will be based on a random lottery system that would take 20-year-olds first.

In a 62-page report in Congress on President Carter's decision to call for the resumption of draft registration for men and his request for authority to require registration of women, the administration stressed that Carter "has no intention under present circumstances" of also calling for renewed military conscription.

Nevertheless, the administration outlined a plan for possible future draft and estimated that 100,000 draftees could be processed by the Selective Service System 28 days after a general military mobilization.

Currently, Carter has authority to require registration for the draft of men between the ages of 18 and 26. He has asked Congress for $45 million to begin registering 19- and 20-year-olds born in 1960 and 1961, and also for authority to require women born in the same years to register.

The president would need congressional approval before the draft could be resumed.

If the draft becomes necessary, according to the report, the lottery system would first take young people who reach the age of 20 the year military conscription is resumed -- those born in 1960 if the draft were resumed this year. The lottery would be based on dates of birth chosen by random selection.

The administration said the registeration of 19- and 20-year-olds this year would produce a sufficiently large manpower pool and noted that Carter's plan calls for the routine registration of 18-year-olds on their birthdays, beginning Jan. 1.

The estimate that 100,000 draftees could be processed in 28 days after a general mobilization was based on the assumption that Congress would grant the president authority to resume the draft one day after he asked for it.

According to this timetable, the first inductees would report to their assigned military bases 13 days after the mobilization.

During such a draft, according to the report, inductees would be notified by a Western Union mailgram. The mailgram would order the inductee to report for a physical examination and processing and would provide information on exemption and deferment rights and forms on which to claim an exemption or deferment, as well as the address of his local draft board, which would consider such claims.

Carter has said no draft instituted by his administration would allow such deferments as those for students, which he says have tended in the past to protect the more affluent from military service at the expense of the poor.

The president's registration plan would not involve the issuance of draft cards or the classification of young people by marital or other status. The administration reported that it had considered requiring classification at the time of registration but had concluded that it would not be worth the cost and would not speed up the process.

"The benefit of classification before mobilization would not be in response time, but in a more orderly induction process since orders would be issued only to those already classified." the report said.

The Carter plan also does not call for reactivating local draft boards at this time. However, the report said that the Selective Service System will work with the nation's governors over the next 18 months "to develop a process for recruiting and training members of local boards" should the resumption of the draft become necessary.

The administration also was required by a congressional resolution to consider the possibility of instituting a system of either voluntary or compulsory national service outside the military. The report concluded that a compulsory system, under which young people chosen by lottery for military service would be offered the option of civilian service, was "the most promising" of the possible plans studied.

It said such a proposal "should be examined further in contingency planning for any actual military conscription," but added that "there is no need for a national civilian service program at this time."