Leaders of the AFL-CIO, National Conference of Catholic Charities, United Auto Workers and National Organization for Women lashed out at the Carter administration yesterday as a coalition of 115 national organizations was formed to fight proposed Social Security benefit cuts.

Former health, education and wefare secretary Wilbur J. Cohen is head of the new coalition, which is called Save Our Security. Cohen told a press conference," I do not intend to see our Social Security system dismantled after 45 years of effort to build it to where it is." He hinted at demonstrations and picketing against HEW Secretary Joseph A. Califano Jr.

Cohen said "we have already won Round one" because House Ways and Means Committee chairman Al Ullman (D-Ore.) has announced he does not expect action on most of the Carter proposals this year. "But that is only Round One in a 15-round battle," said Cohen.

Arlie Scott, of the National Organization for Women, said the cuts proposed by Carter would fall heavily on low-income women.

The president's proposals include wiping out the $122 floor for monthly benefits, phasing out student benefits paid to orphans ages 18 to 21 if they are in college, reducing disability benefits, cutting off the mother's benefit when her youngest child reaches 16 (instead of 18), and dropping the $255 lump-sum burial payment.

Former Social Security commissioner Robert M. Ball said it isn't necessary to cut benefits in order to hold down scheduled increases in payroll taxes. For example, he said, the payroll tax rate is scheduled to go from 6.13 percent to 6.65 percent in 1981, but half the increase could be eliminated if hospital cost-containment legislation is passed.