The YWCA, other girls' clubs and several major organizations serving hundreds of thousands of youths and families yesterday attacked a Reagan administration proposal to require that parents be notified when their children receive prescription contraceptives.

The protests came amid preliminary reports that some teen-agers may be staying away from family-planning clinics in California, in part because of mistaken "fear the proposal has already been implemented," said Dr. Elizabeth Johns of the National Council of Negro Women.

She cautioned, however, that "it is clearly too soon to suggest these isolated data indicate a national trend." A Planned Parenthood spokesman said there had been similiar "scattered reports" throughout the country.

Calling the draft regulation "counterproductive," Johns contended that it was a "squeal rule" that would "only aggravate the already serious problem of unintended adolescent pregnancy."

Johns' concern was shared by representatives of the YWCA, Girls Clubs of America, the National Urban League, the Union of American Hebrew Congregations and the United Presbyterian Church in the U.S.A., all of which support voluntary family involvement in matters of adolescent sexuality.

Yesterday's news conference was part of a meeting on teen-age pregnancy sponsored by the Center for Population Options and Columbia University's Center for Population and Family Health.

Others, including those active against abortion, have said the proposal does not go far enough and say parental consent should be required.

The Department of Health and Human Services' proposal, published Feb. 22, applies to federally funded clinics, and would affect girls under 18. The government argues that notices should be sent to parents within 10 days after services are provided because of the possible health effects of contraceptive drugs and devices. A 60-day public comment period is under way.

Johns said that at least two California Planned Parenthood affiliates, in Contra Costa and San Francisco, reported fewer new patients since January press accounts of the notification proposal.