Leaders of 20 national black and Hispanic organizations said yesterday they are strongly opposed to convening a constitutional convention to propose a balanced-budget amendment.

"That kind of convention could start with an attempt to blance the budget and end with the repeal of the Bill of Rights," said Raul Yzaguirre, president of the National Council of La Raza, a Washington-based civil rights group for Spanish-speaking people.

Yzaguirre's comment brought laughter yesterday at a press conference called by the National Working Committee on Concerns of Hispanics and Blacks. But it was made in seriousness.

Despite a Louis Harris poll last week showing that America has become more liberal in its racial attitudes, Yzaguirre and colleagues from groups such as the National Urban Coalition and the National Association for the Advancement of Colored People said they fear a convention could lead to a reexamination of civil rights legislation.

Asked if that attitude reflected paranoia on the part of minority leaders, National Urban Coalition President M. Carl Holman said, "Yes. Paranoia is a sign of health amongst the minorities."

Holman said minority groups represented by the Hispanic-black working committee have healthy suspicions about proposed administration budget cuts in job training and social programs.

The groups agreed to work together against the proposed cuts, he said.

"We agreed that from all that the administration itself has said, the proposed budget cuts in social programs would make a minuscule difference in federal savings," Holman said.

The group also agreed yesterday to work for:

Elimination of "unnecessary friction" between blacks and Hispanics on the issue of illegal Mexican immigrants. Black committee members here said they would refrain from comment on the issue until further conferences with their Hispanic conterparts.

Support of bilingual and bicultural programs for Spanish-speaking students.

The committee also agreed to ask for a joint meeting with President Carter to duscuss its concerns.