Mayor Marion Barry voiced opposition for the first time yesterday to plans by the federal government and the D.C. Coalition for the Homeless to open a temporary, 600-bed shelter in Anacostia, saying that the Southeast Washington area should not be used as a "dumping ground."

Barry, speaking to reporters at his monthly news conference, said he was preparing a letter to Health and Human Services Secretary Margaret M. Heckler stating his opposition to "that crazy plan of theirs that doesn't solve the problems in the short run or the long run either."

"First of all," Barry said, "Anacostia is not a good place to put that many people. Secondly, it is doubtful that a number of the homeless . . . will get on the buses and go over there. So we'll have them on the street again. I predict that some of the people . . . may be left out in the cold and freeze. I am opposed to it."

The Department of Health and Human Services and the coalition unveiled a plan Sept. 26 to move homeless men from a shelter at 425 Second St. NW, operated by the Community for Creative Non-Violence, to an old Navy Department building at 1900 Anacostia Dr. SE.

The $3.7 million proposal, which is under review by the U.S. Court of Appeals for the District, calls for the coalition to use the Navy building temporarily, and subsequently to develop smaller, long-term shelters for men and women.

James Hunter, acting deputy undersecretary for intergovernmental affairs at HHS, declined to comment on the mayor's strong opposition to the proposal until the department receives Barry's letter.

Last fall, President Reagan promised CCNV to provide funds to convert the Second Street facility to a model shelter, but in June the administration and Mitch Snyder, a CCNV leader, clashed over whether the government was making good on its promise. Barry has repeatedly said the shelter dispute is not his problem and has declined to cooperate with federal efforts to establish an alternative shelter.

Barry's opposition to an earlier federal proposal, to house the homeless in trailers placed in various locations around the District, effectively scotched that plan and forced HHS officials to come up with the Navy building plan.

"It was the federal government that made this commitment. I didn't," Barry said. "I don't think I should be held responsible for something that President Reagan did . . . . We are going to let the federal government stew in its own stew."

Barry also complained yesterday that a $3.7 million federal grant to the coalition would not be enough to sustain the shelter program and warned that the District government would not be responsible for providing more funds later.

"I guarantee after 18 months, the Coalition for the Homeless will come back to the city government and ask for some money," he said.

Asked whether he thought the District's emergency shelter program director, Dennis Bethea, who also serves on the board of the coalition, would face a conflict of interest if the coalition did ask for additional funds, Barry said, "I think your facts are not quite accurate about Mr. Bethea. He is a D.C. government employe. He is not a member of the coalition."

However, coalition spokesman Boyd Smith later confirmed that Bethea does serve on the board but said Bethea "has made clear . . . he would resign" from the homeless advocacy group if it ever went to the city for additional funding. Bethea could not be reached for comment.

On another subject, Barry said that he opposes a November referendum, backed by tenant groups, that would alter the city's new rent control law. "I don't believe you ought to run the government by referendum," the mayor said.