President Reagan's proposed cutbacks in federal aid to local governments would "eliminate or sharply reduce nearly every federal urban program of significant benefit to cities," a study released last week said.

The report, compiled by the United States Conference of Mayors, said Reagan's $974 billion fiscal 1986 budget proposal would slash grants to cities and states by $20 billion, an 18 percent decrease.

In addition, if the budget is enacted, federal assistance to cities would drop to 5.5 percent of federal outlays, compared with 8.5 percent in 1981.

Among the group's conclusions:

* Elimination of the $4.6 billion General Revenue Sharing program "would trigger wide-ranging and disruptive cuts in the most basic public services, such as police and fire protection, and would force some local tax increases."

Ninety-seven percent of 157 cities responding to the survey said they would be forced to cut services, and 68 percent said they would need to increase local revenues.

* Low income and elderly citizens would be hard hit by cuts in federal housing aid from nearly $11 billion this year to less than $500 million next year.

* Public transit would be slashed by $2.75 billion, resulting in "service cuts and some shutdowns." Of cities responding, 83 percent said transit fares would have to increase if federal assistance is cut. Nearly 39 percent said their systems would be threatened with shutdowns.

* Elimination of the $400 million in Urban Development Action Grants would halt progress on 96 percent of planned projects scheduled to receive such aid. The mayors conference calls the grants "tested, effective tools to leverage large amounts of private development in our cities," and estimated that 102,000 jobs and $4.3 billion in investment would be lost.

* Aid to provide summer employment for disadvantaged youths would be cut by more than $1 billion, resulting in the loss of 37,000 jobs.

* "Despite increasing numbers of hungry and homeless citizens, and despite increasing demand for emergency human services, there would be no funds to continue the emergency food and shelter programs."