The coalition representing 28 federal and postal unions, professional groups and retiree organizations announced yesterday that it will oppose an Office of Personnel Management plan to set up a new retirement program -- featuring special tax shelters -- for government employes.

The Fund for Assuring an Independent Retirement (FAIR) said OPM's plan would mean lower retirement benefits for most government workers than the present civil service retirement system.

Under OPM's plan, each employe would have a special retirement account. The government would contribute an amount of money each year equal to 11 percent of an employe's salary. Workers would not pay anything into the system. They would, however, have the option of investing up to $5,000 of their own money in individual retirement accounts to supplement their regular civil service pensions. They would also pay into, and be covered by, Social Security.

Employes could not withdraw money from their retirement accounts until they were at least 59 1/2 years old.

OPM's plan, if passed by Congress, would cover everyone hired by the government since January 1984. Employes hired before then, who are under the regular civil service system, could stick with their current system, or come into the new plan with an initial investment equal to all the money they have contributed to the retirement fund, plus matching payments from the government. That would be 14 percent of their total salary and OPM says interest would be added to that initial deposit.

But FAIR officials say OPM's plan would harm middle- and low-income employes, giving them smaller pensions than they could expect under the present system. It guarantees workers a percentage of their pre-retirement income as a pension, based on salary and service time.

OPM's plan would not have a defined benefit; rather it would base annuities on investments and the interest paid on them.

FAIR officials say OPM's draft bill pegs interest on the retirement account and optional investment packages at the rate for special issue T-bills. They now run about 7 percent a year. FAIR claims that the special-issue interest historically barely keeps pace with inflation, so that lifetime earnings on accounts would be relatively low.