The nation's black leaders opened the Conference on a Black Agenda for the 80s today with the Rev. Jessie Jackson issuing a blistering attack on all presidential candidates for evading crucial domestic issues and appealing to the politicians to "stop clowning."

Jackson, president of the Chicago-based Operation PUSH and a long-time civil rights activist, likened the current presidential campaign to a circus during an opening address to the conference. "The presidential campaign to date is a three-ring circus of Democrats, Republicans and the medi," he said. "The people have been locked out."

Conference organizers had hoped their meeting would attract the major political candidates for a discussion of unemployment, housing, inflation and affirmative action. But tonight Richard Hatcher, head of the conference and mayor of Gary, Ind., confirmed rumors that had circulated all day and announced that none of the candidates will appear.

Jackson's speech and the cancellation of the candidate forum seemed to reinforce the sense of guarded pesimism and urgency about combating eroding civil rights that many of the more than 1,200 leaders gathering here have expressed.

But former U.N. ambassador Andrew Young, an outspoken supporter of President Carter, added a positive note tonight by saying blacks have made significant gains in the 1970s in "quiet ways," although he said much more needs to be done to gain economic progress for blacks.

"We know the story of John Kennedy" Young said. "We tend to forget the times we were asked to call off the demonstration because it would have been politically embarrassing. He did finally come through with an agenda, and by his blood we did achieve a measure of freedom."

Young said "there is a measure of mature awareness among black leadership that no one candidate is going to be any better than the people who put him in office. No one will be better than the coalition that surrounds him. Leaders have become great because of the challenges."

Young, who received polite but unenthusiastic applause, said "things ain't nearly as bad as some people would like to make them. We are attempting to overcome the legacy and scars of years of oppression. We must kindle the fires of hope in our people?"

Earlier, Rep. Cardis Collins (D-Ill.), chairwoman of the Congressional Black Caucus and national coordinator for the conference, outlined the goal of the meetings here by saying: "We have gathered for the purpose of defining our goals and objectives for the next 10 years. We are organizing and building the infrastructure to enable us to exercise formidable political power."

While the conference includes many elected and appointed black officials who have declared their support for a presidential candidate, the conference is supposed to devise an agenda by which all the candidats can be judged.

"I have grave concerns because many of us have come here already locked into candidates," Collins said today. "It is time for all of us to reevaluate where we are and where we are going and to realize that what happens in the 1980s is going to affect our lives for years to come.

"Some have said they are favoring this candidate or they are favoring that candidate, but there is no use for squabbling and arguing over who the slavemaster is going to be," she said to thunderous applause.

While some delegates said they understood the necessity to clarify important issues through adoption of a national agenda, some, remembering past national black conferences, have expressed doubts privately over what the meeting can accomplish. From the start the conference has been beset with confusion, which has been compounded by simple logistical problems of finding appropriate meeting rooms.

But Franklin Hightower, from Decatur, Ga., a delegate of Lobbying America, saw the issue differently. "This agenda is a benchmark that we will have to meet as well," he said. "We made it. We're making a presentation of the needs. We're not saying to America you all do it for us."

"All of us have our own visions and our own agendas," said PUSH's Jackson. "But we have the good judgment to come here to reach a consensus. Our job is to determine what the agenda should be and then send each [organization] back to disseminate the information about our agenda."

The leaders meeting here are officers of the major black social, political and religious organizations -- the Elks, Masons, fraternities and sororities, religious denominations as well as the National Conference of Black Mayors, the National Association for the Advancement of Colored People, the National Urban League, black legal associations and other professional organizations.

Each of these leaders is supposed to take the agenda developed during the three-day meeting here back to their communities, giving national impact to the policies.