Mellon Bank Corp., the nation's 11th largest bank-holding company, is negotiating with the state of Maryland to acquire deposits of Community Savings and Loan and reopen the crippled Bethesda thrift as a new institution, Mellon officials and Gov. Harry Hughes announced today.

The Pittsburgh-based bank has been examining the assets and facilities of Community for three weeks, spokesmen said, with a view toward reopening at least some of the thrift association's nine branches as offices of the new Mellon Bank Maryland.

If the agreement goes through, it would mark the second time in less than a year that a giant, out-of-state bank has moved into the lucrative Washington market by purchasing a faltering Maryland savings and loan.

"It's too early to predict whether an agreement will be reached, but the up side for Mellon would be the very attractive market" that Community represents, said Barry I. Deutsch, a senior vice president at Mellon and the bank's marketing director.

Community, which has 27,000 customers, maintained branches in Howard, Montgomery, and Prince George's counties until it was placed in state conservatorship Sept. 5. By that time, its far-flung real estate empire, known as EPIC, had collapsed. Since then, Community customers have had virtually no access to their $332 million in deposits.

Community, which has a $162 million debt, and the state-controlled First Maryland and Old Court savings and loans represent an enormous financial liability for a state government that has promised to reimburse depositors. In a related development today, Hughes announced his opposition to a legislative proposal that would reduce that liability by limiting the amount of money returned to some thrift depositors.

Mellon's presence in Maryland is similar to the early stages of Chase Manhattan Corp.'s successful effort last year to acquire Friendship, Chesapeake and Merritt Commercial savings and loans. In that transaction, which required the approval of the General Assembly, the state government paid the New York bank $25 million to take the three thrift associations off its hands, and Chase reopened them as the new Chase Bank of Maryland.

Hughes has said he is prepared to offer a sweetener to Community's buyer of at least $147 million. The legislature also would have to pass a measure authorizing the state government to retain some Community assets, an aide to Hughes said today.

Mellon Bank Corp., a $33.4 billion concern that owns four banks in Pennsylvania and one in Delaware, is the second institution to express an interest in acquiring Community. Meritor Savings Bank of Arlington, a subsidiary of a Philadelphia financial services company, also has announced its intention to acquire Community, but that deal has been slowed for weeks by a series of federal requirements.

Hughes characterized both Meritor and Mellon today as "bona fide suitors" of Community. Administration officials said tonight that negotiations with the two outside banks should be concluded within three weeks.

Hughes joined House Speaker Benjamin L. Cardin (D-Baltimore) today in assailing the bill pending in a House committee that would restructure the state's insurance of accounts at Community, Old Court, and First Maryland.

The bill, aimed at the wealthiest of depositors, would reduce the state's insurance liability to $100,000 per depositor instead of the $100,000 per account that Hughes has pledged. Some depositors maintain more than one account; state officials estimated the bill would affect 2,500 such accounts.

The governor, at his weekly news conference, said that it would be unfair for the state to change its mind now and that he would be "inclined to veto" it should it reach his desk.

"This is a commitment that a lot of people have relied upon," he said.

"To renege on that now sends a bad message insofar as the credibility and the trustworthiness of the state of Maryland in living up to its commitment."

Support for the measure, drafted by Del. Terry Connelly (D-Baltimore County), shifted today as Senate President Melvin A. Steinberg (D-Baltimore County), who indicated Wednesday that he would support it, called it "simplistic" and said that he would instead support an administration bill that protects the state from suits by depositors who hold accounts greater than $100,000.

Revised figures compiled by the legislature's Department of Fiscal Services and released today show that the state would save $76.7 million by insuring per depositor rather than per account.

An attorney general's opinion also released today gives the General Assembly the option to choose which way it wishes to insure those accounts.

Del. Patricia Sher (D-Montgomery), a member of the committee considering the bill, believes it should be approved. "I can cry for poor people who've got their life savings tied up," she said, "but I can't cry for somebody who's got $600,000 in six different accounts at any savings and loan, particularly when my constituents have to pay for it."