Twenty-one persons were indicted on fraud charges here yesterday for allegedly receiving unemployment benefits while working for the National Park Service as part of a major effort to use computers for catching illegal "double-dippers" on the federal payroll, U.S. Attorney Joseph E. diGenova announced.

The indictments, filed by a grand jury in U.S. District Court, were the result of a computer cross-match between lists of Park Service workers and those getting D.C. unemployment compensation, diGenova said.

Similar computer checks have found government workers who were fraudulently receiving welfare benefits and others who had defaulted on student loans.

During the last two years, diGenova said, 27 local Postal Service employes have been convicted of unemployment compensation fraud after having been caught by the computers.

"Additional cases involving similar alleged false claims by federal employes are currently under review," diGenova said. He said he anticipates further indictments from other agencies in coming months.

Those indicted yesterday were seasonal workers who continued to sign claim forms, declaring they were out of work, after they had returned to their Park Service jobs, prosecutors said. Altogether, they obtained about $54,000 in D.C. unemployment benefits to which they were not entitled between 1980 and last year, the indictment charged. Seventeen are D.C. residents; four live in Prince George's County.

Prosecutors said most are laborers, although one is a supervisor and several worked in skilled trades. Sixteen are currently employed by the Park Service, one works for another federal agency, and four have left government service.

According to the indictments, the amount of benefits fraudulently received ranged from $900 to $5,541.

Each of those indicted was charged with one count of mail fraud, which carries a maximum penalty of five years in prison and a $1,000 fine, and between five and 23 counts of making false claims. Each of these counts has a maximum penalty of five years in prison and a $10,000 fine.